javascript : Interview Questions

Answer:

JavaScript is a client-side as well as server side scripting language that can be inserted into HTML pages and is understood by web browsers. JavaScript is also an Object based Programming language

Answer:

Java is a complete programming language. In contrast, JavaScript is a coded program that can be introduced to HTML pages. These two languages are not at all inter-dependent and are designed for the different intent. Java is an object - oriented programming (OOPS) or structured programming language like C++ or C whereas JavaScript is a client-side scripting language.

Answer:

Following are the JavaScript Data types:

  • Number
  • String
  • Boolean
  • Object
  • Undefined

Answer:

isNan function returns true if the argument is not a number otherwise it is false.

Answer:

JavaScript is faster. JavaScript is a client-side language and thus it does not need the assistance of the web server to execute. On the other hand, ASP is a server-side language and hence is always slower than JavaScript. Javascript now is also a server side language (nodejs).

Answer:

Negative Infinity is a number in JavaScript which can be derived by dividing negative number by zero.

Answer:

Breaking within a string statement can be done by the use of a backslash, '\', at the end of the first line
 

[removed]("This is \a program");

And if you change to a new line when not within a string statement, then javaScript ignores break in line.
 
var x=1, y=2,
z=
x+y;

The above code is perfectly fine, though not advisable as it hampers debugging.

Answer:

Netscape is the software company who developed JavaScript.

Answer:

Undeclared variables are those that do not exist in a program and are not declared. If the program tries to read the value of an undeclared variable, then a runtime error is encountered.

Undefined variables are those that are declared in the program but have not been given any value. If the program tries to read the value of an undefined variable, an undefined value is returned.

Answer:

<html> 
<head> 
<title>t1</title> 
<script type="text/javascript"> 
	function addNode() { var newP = document.createElement("p"); 
	var textNode = document.createTextNode(" This is a new text node"); 
	newP.appendChild(textNode); document.getElementById("firstP").appendChild(newP); } 
</script> </head> 
<body> <p id="firstP">firstP<p> </body> 
</html>

Answer:

Global variables are those that are available throughout the length of the code, that is, these have no scope. The var keyword is used to declare a local variable or object. If the var keyword is omitted, a global variable is declared.

Example:

// Declare a global globalVariable = "Test";

The problems that are faced by using global variables are the clash of variable names of local and global scope. Also, it is difficult to debug and test the code that relies on global variables.

Answer:

A prompt box is a box which allows the user to enter input by providing a text box. Label and box will be provided to enter the text or number.

Answer:

=== is called as strict equality operator which returns true when the two operands are having the same value without any type conversion.

Answer:

'This' keyword refers to the object from where it was called.

Answer:

To submit a form using JavaScript use document.form[0].submit();

document.form[0].submit();
 

Answer:

Timers are used to execute a piece of code at a set time or also to repeat the code in a given interval of time. This is done by using the functions setTimeout, setInterval and clearInterval.

The setTimeout(function, delay) function is used to start a timer that calls a particular function after the mentioned delay. The setInterval(function, delay) function is used to repeatedly execute the given function in the mentioned delay and only halts when cancelled. The clearInterval(id) function instructs the timer to stop.

Timers are operated within a single thread, and thus events might queue up, waiting to be executed.

Answer:

Yes JavaScript does support automatic type conversion, it is the common way of type conversion used by JavaScript developers

Answer:

// for Single line comments and

/* Multi

Line

Comment

*/

Answer:

It can be done in the following way:

document.getElementById("myText").style.fontSize = "20";
or
document.getElementById("myText").className = "anyclass";
 

Answer:

'ViewState' is specific to a page in a session.

'SessionState' is specific to user specific data that can be accessed across all pages in the web application.

Answer:

There are two ways to read and write a file using JavaScript

  • Using JavaScript extensions
  • Using a web page and Active X objects

Answer:

Following are looping structures in Javascript:

  • For
  • While
  • do-while loops

Answer:

Variable typing is used to assign a number to a variable and the same variable can be assigned to a string.

i = 10;
i = "string";
This is called variable typing.

Answer:

The parseInt() function is used to convert numbers between different bases. parseInt() takes the string to be converted as its first parameter, and the second parameter is the base of the given string.

In order to convert 4F (of base 16) to integer, the code used will be -
 

parseInt ("4F", 16);

Answer:

"==" checks only for equality in value whereas "===" is a stricter equality test and returns false if either the value or the type of the two variables are different.

Answer:

Since 3 and 2 are integers, they will be added numerically. And since 7 is a string, its concatenation will be done. So the result would be 57.

Answer:

In order to detect the operating system on the client machine, the navigator.platform string (property) should be used.

Answer:

The NULL value is used to represent no value or no object. It implies no object or null string, no valid boolean value, no number and no array object.

Answer:

The delete keyword is used to delete the property as well as its value.

var student= {age:20, batch:"ABC"};
delete student.age;
 

Answer:

Undefined value means the

  • Variable used in the code doesn't exist
  • Variable is not assigned to any value
  • Property doesn't exist

Answer:

  • Alert
  • Confirm and
  • Prompt

Answer:

Void(0) is used to prevent the page from refreshing and parameter "zero" is passed while calling.

Void(0) is used to call another method without refreshing the page.

Answer:

The following code has to be inserted to achieve the desired effect:

 

Answer:

All variables in the JavaScript are object data types.

Answer:

An alert box displays only one button which is the OK button.

But a Confirmation box displays two buttons namely OK and cancel.

Answer:

Escape characters (Backslash) is used when working with special characters like single quotes, double quotes, apostrophes and ampersands. Place backslash before the characters to make it display.

document.write "I m a "good" boy"
document.write "I m a \"good\" boy"
 

Answer:

Cookies are the small test files stored in a computer and it gets created when the user visits the websites to store information that they need. Example could be User Name details and shopping cart information from the previous visits.

Answer:

The pop() method is similar as the shift() method but the difference is that the Shift method works at the start of the array. Also the pop() method take the last element off of the given array and returns it. The array on which is called is then altered.

var cloths = ["Shirt", "Pant", "TShirt"];
cloths.pop();
//Now cloth becomes Shirt,Pant

Answer:

No. JavaScript does not have concept level scope. The variable declared inside the function has scope inside the function.

Answer:

If you use innerHTML in JavaScript the disadvantage is

  • Content is replaced everywhere
  • We cannot use like "appending to innerHTML"
  • Even if you use +=like "innerHTML = innerHTML + 'html'" still the old content is replaced by html
  • The entire innerHTML content is re-parsed and build into elements, therefore its much slower
  • The innerHTML does not provide validation and therefore we can potentially insert valid and broken HTML in the document and break it

Answer:

Break statement exits from the current loop.

Continue statement continues with next statement of the loop.

Answer:

They are as –

  • Primitive
  • Reference types.
Primitive types are number and Boolean data types. Reference types are more complex types like strings and dates.

Answer:

Generic objects can be created as:

var I = new object();
 

Answer:

'Typeof' is an operator which is used to return a string description of the type of a variable.

Answer:

Try… Catch---finally is used to handle exceptions in the JavaScript

Try{
	Code
}
Catch(exp){
	Code to throw an exception
}
Finally{
	Code runs either it finishes successfully or after catch
}
 

Answer:

document.write("Welcome") is used to print the text – Welcome in the screen.

Answer:

Blur function is used to remove the focus from the specified object.

Answer:

There are three types of errors:

  • Load time errors: Errors which come up when loading a web page like improper syntax errors are known as Load time errors and it generates the errors dynamically.
  • Run time errors: Errors that come due to misuse of the command inside the HTML language.
  • Logical Errors: These are the errors that occur due to the bad logic performed on a function which is having different operation.

Answer:

The push method is used to add or append one or more elements to the end of an Array. Using this method, we can append multiple elements by passing multiple arguments

Answer:

Unshift method is like push method which works at the beginning of the array. This method is used to prepend one or more elements to the beginning of the array

Answer:

Both are almost similar. JavaScript is developed by Netscape and Jscript was developed by Microsoft .

Answer:

Properties are assigned to objects in the following way -

obj["class"] = 12;
or
obj.class = 12;
 

Answer:

Strict Mode adds certain compulsions to JavaScript. Under the strict mode, JavaScript shows errors for a piece of codes, which did not show an error before, but might be problematic and potentially unsafe. Strict mode also solves some mistakes that hamper the JavaScript engines to work efficiently.

Strict mode can be enabled by adding the string literal "use strict" above the file. This can be illustrated by the given example:
 

function myfunction() {
    "use strict";
    var v = "This is a strict mode function";
}

Answer:

The status can be acquired as follows -

alert(document.getElementById('checkbox1').checked);

If the CheckBox will be checked, this alert will return TRUE.

Answer:

The eval() function evaluates JavaScript code represented as a string. The string can be a JavaScript expression, variable, statement, or sequence of statements.
 

console.log(eval('1 + 2')); //  3
 

Answer:

Below are the main differences between window and document,
 

Window Document
It is the root level element in any web page It is the direct child of the window object. This is also known as Document Object Model(DOM)
By default window object is available implicitly in the page You can access it via window.document or document.
It has methods like alert(), confirm() and properties like document, location It provides methods like getElementById, getElementByTagName, createElement etc

Answer:

The window.history object contains the browser's history. You can load previous and next URLs in the history using back() and next() methods.
 

function goBack() {
  window.history.back()
}
function goForward() {
  window.history.forward()
}

Note: You can also access history without window prefix.

Answer:

Closure is a locally declared variable related to a function which stays in memory when the function has returned.

function greet(message) {

    console.log(message);

}

function greeter(name, age) {

    return name + " says howdy!! He is " + age + " years old";

}

// Generate the message

var message = greeter("James", 23);

// Pass it explicitly to greet

greet(message);

This function can be better represented by using closures

function greeter(name, age) {

    var message = name + " says howdy!! He is " + age + " years old";

    return function greet() {

        console.log(message);

    };

}

// Generate the closure

var JamesGreeter = greeter("James", 23);

// Use the closure

JamesGreeter();
 

Answer:

A value can be appended to an array in the given manner -

arr[arr.length] = value;

Answer:

The for-in loop is used to loop through the properties of an object.

The syntax for the for-in loop is -
 

for (variable name in object){
	statement or block to execute
}
In each repetition, one property from the object is associated to the variable name, and the loop is continued till all the properties of the object are depleted.

Answer:

A function that is declared without any named identifier is known as an anonymous function. In general, an anonymous function is inaccessible after its declaration.

Anonymous function declaration -
 

var anon = function() {
	alert('I am anonymous');
};
anon();

Answer:

The function .call() and .apply() are very similar in their usage except a little difference. .call() is used when the number of the function's arguments are known to the programmer, as they have to be mentioned as arguments in the call statement. On the other hand, .apply() is used when the number is not known. The function .apply() expects the argument to be an array.

The basic difference between .call() and .apply() is in the way arguments are passed to the function. Their usage can be illustrated by the given example.
 

var someObject = {
myProperty : 'Foo',

myMethod : function(prefix, postfix) {

	alert(prefix + this.myProperty + postfix);
}
};
someObject.myMethod('<', '>'); // alerts ''
var someOtherObject  = {

	myProperty : 'Bar'

};
someObject.myMethod.call(someOtherObject, '<', '>'); // alerts ''

someObject.myMethod.apply(someOtherObject, ['<', '>']); // alerts ''

Answer:

JavaScript allows DOM elements to be nested inside each other. In such a case, if the handler of the child is clicked, the handler of parent will also work as if it were clicked too.

Answer:

Yes, JavaScript is case sensitive. For example, a function parseInt is not same as the function Parseint.

Answer:

The 'And' Operator (&&), 'Or' Operator (||) and the 'Not' Operator (!) can be used in JavaScript.

*Operators are without the parenthesis.

Answer:

This can be done by including the name of the required frame in the hyperlink using the 'target' attribute.

>New Page
 

Answer:

Break statement is used to come out of the current loop while the continue statement continues the current loop with a new recurrence.

Answer:

Both web-garden and web-farm are web hosting systems. The only difference is that web-garden is a setup that includes many processors in a single server while web-farm is a larger setup that uses more than one server.

Answer:

Assigning properties to objects is done in the same way as a value is assigned to a variable. For example, a form object's action value is assigned as 'submit' in the following manner - Document.form.action="submit"

Answer:

This can be done by Using JavaScript extensions (runs from JavaScript Editor), example for opening of a file -

fh = fopen(getScriptPath(), 0);
 

Answer:

DOM stands for Document Object Model and is responsible for how various objects in a document interact with each other. DOM is required for developing web pages, which includes objects like paragraph, links, etc. These objects can be operated to include actions like add or delete. DOM is also required to add extra capabilities to a web page. On top of that, the use of API gives an advantage over other existing models.

Answer:

Events are the actions that result from activities, such as clicking a link or filling a form, by the user. An event handler is required to manage proper execution of all these events. Event handlers are an extra attribute of the object. This attribute includes event's name and the action taken if the event takes place.

Answer:

By default, the parsing of the HTML code, during page loading, is paused until the script has not stopped executing. It means, if the server is slow or the script is particularly heavy, then the webpage is displayed with a delay. While using Deferred, scripts delays execution of the script till the time HTML parser is running. This reduces the loading time of web pages and they get displayed faster.

Answer:

The different functional components in JavaScript are-

First-class functions: Functions in JavaScript are utilized as first class objects. This usually means that these functions can be passed as arguments to other functions, returned as values from other functions, assigned to variables or can also be stored in data structures.

Nested functions: The functions, which are defined inside other functions, are called Nested functions. They are called 'everytime' the main function is invoked.

Answer:

Screen objects are used to read the information from the client's screen. The properties of screen objects are -

  • AvailHeight: Gives the height of client's screen
  • AvailWidth: Gives the width of client's screen.
  • ColorDepth: Gives the bit depth of images on the client's screen
  • Height: Gives the total height of the client's screen, including the taskbar
  • Width: Gives the total width of the client's screen, including the taskbar

Answer:

This method is functional at the starting of the array, unlike the push(). It adds the desired number of elements to the top of an array.

var name = [ "john" ];
name.unshift( "charlie" );
name.unshift( "joseph", "Jane" );
console.log(name);
The output is shown below:
[" joseph "," Jane ", " charlie ", " john "]
 

Answer:

The escape () function is responsible for coding a string so as to make the transfer of the information from one computer to the other, across a network.

Output: Hello%3F%20How%20are%20you%21

The unescape() function is very important as it decodes the coded string.

It works in the following way. 
 

Output: Hello? How are you!

Answer:

EncodeURl() is used to convert URL into their hex coding. And DecodeURI() is used to convert the encoded URL back to normal.

Output -

my%20test.asp?name=st%C3%A5le&car=saab

my test.asp?name=ståle&car=saab

Answer:

innerHTML content is refreshed every time and thus is slower. There is no scope for validation in innerHTML and, therefore, it is easier to insert rouge code in the document and, thus, make the web page unstable.

Answer:

ECMA Script are like rules and guideline while Javascript is a scripting language used for web development.

Answer:

Namespacing is used for grouping the desired functions, variables etc. under a unique name. It is a name that has been attached to the desired functions, objects and properties. This improves modularity in the coding and enables code reuse.

Answer:

In JavaScript, if you try to use a variable that doesn't exist and has not been declared, then JavaScript will throw an error var name is not defined and script will stop executing. However, if you use typeof undeclared_variable, then it will return undefined.

Before getting further into this, let's first understand the difference between declaration and definition.

Let's say var x is a declaration because you have not defined what value it holds yet, but you have declared its existence and the need for memory allocation.
 

var x; // declaring x
> console.log(x); //output: undefined 
 

Here var x = 1 is both a declaration and definition (also we can say we are doing an initialisation). In the example above, the declaration and assignment of value happen inline for variable x. In JavaScript, every variable or function declaration you bring to the top of its current scope is called hoisting.

The assignment happens in order, so when we try to access a variable that is declared but not defined yet, we will get the result undefined.
 

var x; // Declaration
if(typeof x === 'undefined') // Will return true

If a variable that is neither declared nor defined, when we try to reference such a variable we'd get the result not defined.
console.log(y);  // Output: ReferenceError: y is not defined
 

Answer:

One of the drawbacks of creating true private methods in JavaScript is that they are very memory-inefficient, as a new copy of the method would be created for each instance.

var Employee = function (name, company, salary) {
    this.name = name || "";       //Public attribute default value is null
    this.company = company || ""; //Public attribute default value is null
    this.salary = salary || 5000; //Public attribute default value is null

    // Private method
    var increaseSalary = function () {
        this.salary = this.salary + 1000;
    };

    // Public method
    this.dispalyIncreasedSalary = function() {
        increaseSlary();
        console.log(this.salary);
    };
};

// Create Employee class object
var emp1 = new Employee("John","Pluto",3000);
// Create Employee class object
var emp2 = new Employee("Merry","Pluto",2000);
// Create Employee class object
var emp3 = new Employee("Ren","Pluto",2500);

Here each instance variable emp1emp2emp3 has its own copy of the increaseSalary private method.

So, as a recommendation, don’t use private methods unless it’s necessary.

Answer:

The best way to find out whether or not an object is an instance of a particular class is to use the toString method from Object.prototype:

var arrayList = [1,2,3];

One of the best use cases of type-checking an object is when we do method overloading in JavaScript. For example, let's say we have a method called greet, which takes one single string and also a list of strings. To make our greet method workable in both situations, we need to know what kind of parameter is being passed. Is it a single value or a list of values?
function greet(param){
 	if(){ // here have to check whether param is array or not 
 	}else{
 	}
}

However, as the implementation above might not necessarily check the type for arrays, we can check for a single value string and put some array logic code in the else block.
function greet(param){
 	if(typeof param === 'string'){ 
 	}else{
 	  // If param is of type array then this block of code would execute
 	}
}


Now it's fine we can go with either of the aforementioned two implementations, but when we have a situation where the parameter can be single valuearray, and object type, we will be in trouble.

Coming back to checking the type of an object, as mentioned previously we can use
Object.prototype.toString
 

if( Object.prototype.toString.call( arrayList ) === '[object Array]' ) {
    console.log('Array!');
}

Answer:

In JavaScript, variable and functions are hoisted. Let's take function hoisting first. Basically, the JavaScript interpreter looks ahead to find all variable declarations and then hoists them to the top of the function where they're declared.

foo(); // Here foo is still undefined 
var foo = function foo(){ 
 	return 12; 
}; 
Behind the scene of the code above looks like this:
var foo = undefined;
    foo(); // Here foo is undefined 
 	   foo = function foo(){
 	      / Some code stuff
      }


var foo = undefined;
 	 foo = function foo(){
 	     / Some code stuff
    }
    foo(); // Now foo is defined here
 

Answer:

Both the Array() and [] work almost the same in JavaScript.If we use them as is (i.e. without any argument) to create an array object, then they will result in an array object of zero length. Even if we pass a string or a list of strings as arguments, the result will be similar.However, they differ when the input argument is of integer type. In that case, the statement will create an uninitialized array of size n. Whereas, the [n] statement will create an array of size 1 and assign n as the value of the first element.

Answer:

  • Class Inheritance: A constructor function instantiates an instance via the "new" keyword. This new instance inherits properties from a parent class.
  • Prototypal Inheritance: An instance is created by cloning an existing object that serves as a prototype. Instances are typically instantiated via factory functions, object literals, or Object.create(). Instances may be composed from many different source objects, allowing for easy selective inheritance.
 

Answer:

Every JavaScript function has a prototype property (by default this property is null), that is mainly used for implementing inheritance. We add methods and properties to a function's prototype so that it becomes available to instances of that function.

Answer:

This is a common practice used in many popular JavaScript libraries (jQuery, Node.js, etc.). It creates a closure around the entire contents of the file which makes a private namespace and thereby avoids potential name clashes between different JavaScript modules and libraries.

Answer:

A function is a piece of code that is called by name and it is not associated with any object, nor defined inside an object. It can be passed data to operate on (i.e. parameter) and can optionally return data (the return value).

/* Function definition*/
function myFunc() {
/* Do some stuff; */
}
myFunc();/* Calling the function */


A method is a piece of code that is called by name and is defined inside an object. It is almost identical to a function except that it is always associated with an object and operating only on data inside it.
 

var methodObject = {
attribute: "xyz",
display: function () { /* Method*/
console.log(this.attribute);
}
}methodObject.display(); /* Calling the method */
 

Answer:

A self-invoking anonymous function runs immediately when we define it and doesn't have a name. E.g.

(function() {
console.log("this will print automatically");
})();
 

Answer:

This may sound trivial and, in fact, it is trivial with ECMAscript 6 which introduces a new Number.isInteger() function for precisely this purpose. However, prior to ECMAScript 6, this is a bit more complicated, since no equivalent of the Number.isInteger() method is provided.

The issue is that, in the ECMAScript specification, integers only exist conceptually; i.e., numeric values are always stored as floating point values.

With that in mind, the simplest and cleanest pre-ECMAScript-6 solution (which is also sufficiently robust to return false even if a non-numeric value such as a string or null is passed to the function) would be the following use of the bitwise XOR operator:
 

function isInteger(x) { return (x ^ 0) === x; } 

The following solution would also work, although not as elegant as the one above:
function isInteger(x) { return (typeof x === 'number') && (x % 1 === 0); }

The following function (or with Math.ceil() or Math.floor() in place of Math.round()) might also seem useful, but the results are not exactly the same as with the above two functions:
function isInteger(x) { return Math.round(x) === x; }

The difference is, these Math-based solutions return true for Infinity and -Infinity, whereas the others (and notably ES6’s Number.isInteger()) return false.

Another fairly common incorrect solution is the following:
function isInteger(x) { return parseInt(x, 10) === x; }

While this parseInt-based approach will work well for many values of x, once x becomes quite large, it will fail to work properly. The problem is that parseInt() coerces its first parameter to a string before parsing digits. Therefore, once the number becomes sufficiently large, its string representation will be presented in exponential form (e.g., 1e+21). Accordingly, parseInt() will then try to parse 1e+21, but will stop parsing when it reaches the e character and will therefore return a value of 1. Observe:
 
String(1000000000000000000000)
'1e+21'
 parseInt(1000000000000000000000, 10)
1
 parseInt(1000000000000000000000, 10) === 1000000000000000000000
false
 

Answer:

var obj = {a: 1 ,b: 2}
var objclone = Object.assign({},obj);
 

Now the value of objclone is {a: 1 ,b: 2} but points to a different object than obj.

Note the potential pitfall, though: Object.clone() will just do a shallow copy, not a deep copy. This means that nested objects aren’t copied. They still refer to the same nested objects as the original:
 

let obj = {
    a: 1,
    b: 2,
    c: {
        age: 30
    }
};

var objclone = Object.assign({},obj);
console.log('objclone: ', objclone);

obj.c.age = 45;
console.log('After Change - obj: ', obj);           // 45 - This also changes
console.log('After Change - objclone: ', objclone); // 45
 

Answer:

var myArray = ['a', 'b', 'c', 'd'];
myArray.push('end');
myArray.unshift('start');
console.log(myArray); // ["start", "a", "b", "c", "d", "end"]

With ES6, one can use the spread operator:
 
myArray = ['start', ...myArray];
myArray = [...myArray, 'end'];

Or, in short:
 
myArray = ['start', ...myArray, 'end'];
 

Answer:

Global variables are available throughout your code: that is, the variables have no scope. Local variables scope, on the other hand, is restricted to where it is declared (like within a function). The var keyword is used to declare a local variable or object, while omitting the var keyword creates a global variable.

Most JavaScript developers avoid globals. One reason why is they're averse to naming conflicts between local and globals, Also, code that depends on globals can be difficult to maintain and test.
 

// Declare a local variable
var localVariable = "YahooBaba"


// Declare a global


globalVariable = "Abc"

Answer:

The key concept here is to get an idea of how the candidate maintains and designs code. Do they design code that is specific to an application with no possible reuse? Do they use class inheritance or the module pattern to build reusable code? These approaches allow multiple developers to work on a project without stepping on their coworkers' toes. In addition, testing modular code or classes is easier to approach than a jumbled mess of code thrown together (look around the Web, and you'll find plenty of examples).

Answer:

Unlike Java or C#, JavaScript is a loosely-typed language (some call this weakly typed); this means that no type declarations are required when variables are created. Strings and numbers can be intermixed with no worries. JavaScript is smart, so it easily determines what the type should be. The types supported in JavaScript are: Number, String, Boolean, Function, Object, Null, and Undefined.

var fName = "Mary";   //Declare a String
var total = 100.32;    //Declare a number


var fName = new String; //Another way to declare a string


fName = "Mary";


var total = new Number;


var isIt = new Boolean;
var names = new Array;
var car = new Object;
 

Answer:

he value of a variable with no value is undefined (i.e., it has not been initialized). Variables can be emptied by setting their value to null. You can test for each using the === (three equal signs) or == (two equal signs) for comparison checking. The big difference is the latter uses coercion, which can have some odd results -- it returns true for a null or undefined comparison if they are either.

if (nullExample === null) { // executes this block only if null }
if (undExample ===Undefined) { // executes this block only if Undefined }


if (bothExampe == null) { // executes this block if Undefined or null }
You can be more exact with a comparison by using the typeof to return an object's type.
If (typeof variable ==="undefined")  { // executes this block of if undefined }
 

Answer:

JavaScript's this keyword normally refers to the object that owns the method, but it depends on how a function is called. Basically, it points to the currently in scope object that owns where you are in the code. When working within a Web page, this usually refers to the Window object. If you are in an object created with the new keyword, the this keyword refers to the object being created. When working with event handlers, JavaScript's this keyword will point to the object that generated the event.

Answer:

Event bubbling describes the behavior of events in child and parent nodes in the Document Object Model (DOM); that is, all child node events are automatically passed to its parent nodes. The benefit of this method is speed, because the code only needs to traverse the DOM tree once. This is useful when you want to place more than one event listener on a DOM element since you can put just one listener on all of the elements, thus code simplicity and reduction. One application of this is the creation of one event listener on a page's body element to respond to any click event that occurs within the page's body.

Answer:

This open-ended question has the potential to spawn a good conversation. There are the vastly popular frameworks like jQuery, although you might be surprised when the person tells you about the framework they developed or even played the contributor role.

The answer to the second question gives you an idea of the candidate's feelings about open source (well, that is the way I see it). There are so many open source options available today (Knockoutpostal.jsjQuery, etc.), and a developer's time is very valuable, so why reinvent the wheel? These open source options provide robust code that has been thoroughly tested by an army of developers. From my perspective, I want developers who will use whatever is available to meet a project's demands. Plus, the interviewee might introduce you to something you've never used.

Answer:

Exceptions that occur at runtime can be handled via try/catch/finally blocks; this allows you to avoid those unfriendly error messages. The finally block is optional, as the bare minimum to use is try/catch. Basically, you try to run code (in the try block between the braces), and execution is transferred to the catch block of code when/if runtime errors occur. When the try/catch block is finally done, code execution transfers to the finally code block. This is the same way it works in other languages like C# and Java.

try {
// do something


} catch (e) {
// do something with the exception
} finally {
// This code block always executes whether there is an exception or not.
}
You can give bonus points to any candidate who discusses the onerror event handler tied to the Window object in the browser -- this allows it to monitor all errors on a page. This allows you to properly handle code syntax errors and runtime exceptions.

Answer:

This subject confuses many developers, and I would expect a candidate to stammer on this question or throw up their hands and say "can anybody?" Instead of trying to explain, take a look at this overview on the Mozilla Developer Network.

Answer:

Timers allow you to execute code at a set time or repeatedly using an interval. This is accomplished with the setTimeout, setInterval, and clearInterval functions. The setTimeout(function, delay) function initiates a timer that calls a specific function after the delay; it returns an id value that can be used to access it later. The setInterval(function, delay) function is similar to the setTimeout function except that it executes repeatedly on the delay and only stops when cancelled. The clearInterval(id) function is used to stop a timer. Timers can be tricky to use since they operate within a single thread, thus events queue up waiting to execute.

Answer:

In JavaScript if you try to use a variable that doesn't exist and has not been declared, then JavaScript will throw an error var name is not defined and the script will stop executing thereafter. But If you use typeof undeclared_variable then it will return undefined.

Before starting further discussion let's understand the difference between declaration and definition.

var x is a declaration because we are not defining what value it holds yet, but we are declaring its existence and the need for memory allocation.
 

var x; // declaring x
console.log(x); // output: undefined

var x = 1 is both declaration and definition, here declaration and assignment of value happen inline for variable x—what we are doing is called "initialisation". In JavaScript both variable declarations and function declarations go to the top of the scope in which they are declared, then assignment happens—this series of events is called "hoisting".

A variable can be declared but not defined. When we try to access it, It will result undefined.
 

var x; // Declaration
typeof x === 'undefined'; // Will return true
A variable can be neither declared nor defined. When we try to reference such variable then the result will be not defined.
console.log(y);  // Output: ReferenceError: y is not defined
 

Answer:

One of the drawbacks of declaring methods directly in JavaScript objects is that they are very memory inefficient. When you do that, a new copy of the method is created for each instance of an object. Here's an example:

ar Employee = function (name, company, salary) {
  this.name = name || "";       
  this.company = company || "";
  this.salary = salary || 5000;

  // We can create a method like this:
  this.formatSalary = function () {
      return "$ " + this.salary;
  };
};

// Alternatively we can add the method to Employee's prototype:
Employee.prototype.formatSalary2 = function() {
    return "$ " + this.salary;
}

//creating objects
var emp1 = new Employee('Yuri Garagin', 'Company 1', 1000000);
var emp2 = new Employee('Dinesh Gupta', 'Company 2', 1039999);
var emp3 = new Employee('Erich Fromm', 'Company 3', 1299483);
In this case each instance variable emp1emp2emp3 has its own copy of theformatSalary method. However the formatSalary2 will only be added once to Employee.prototype.

Answer:

A closure is a function defined inside another function (called parent function) and as such it has access to the variables declared and defined within its parent function's scope.

The closure has access to the variables in three scopes:

  • Variable declared in his own scope
  • Variable declared in its parent function's scope
  • Variable declared in the global namespace
var globalVar = "abc"; //Global variable

// Parent self-invoking function
(function outerFunction (outerArg) { // start of outerFunction's scope

  var outerFuncVar = 'x'; // Variable declared in outerFunction's function scope   
  
  // Closure self-invoking function
  (function innerFunction (innerArg) { // start of innerFunction's scope

    var innerFuncVar = "y"; // variable declared in innerFunction's function scope
    console.log(         
      "outerArg = " + outerArg + "\n" +
      "outerFuncVar = " + outerFuncVar + "\n" +
      "innerArg = " + innerArg + "\n" +
      "innerFuncVar = " + innerFuncVar + "\n" +
      "globalVar = " + globalVar);
  	
  // end of innerFunction's scope
  
  })(5); // Pass 5 as parameter to our Closure

// end of outerFunction's scope

})(7); // Pass 7 as parameter to the Parent function

innerFunction is a closure which is defined inside outerFunction and consequently has access to all the variables which have been declared and defined within outerFunction's scope as well as any variables residing in the program's global scope.

The output of the code above would be:
 

outerArg = 7
outerFuncVar = x
innerArg = 5
innerFuncVar = y
globalVar = abc
 

Answer:

There are a couple of ways by which we can empty an array, So let's discuss all the possible way by which we can empty an array.

Method 1

 
arrayList = [];


The code above will set the variable arrayList to a new empty array. This is recommended if you don't have references to the original array arrayList anywhere else because It will actually create a new empty array. You should be careful with this way of empty the array, because if you have referenced this array from another variable, then the original reference array will remain unchanged, Only use this way if you have only referenced the array by its original variable arrayList.

For instance:

var arrayList = ['a', 'b', 'c', 'd', 'e', 'f']; // Created array
var anotherArrayList = arrayList;  // Referenced arrayList by another variable
arrayList = []; // Empty the array
console.log(anotherArrayList); // Output ['a', 'b', 'c', 'd', 'e', 'f']


Method 2

 
arrayList.length = 0;

The code above will clear the existing array by setting its length to 0. This way of emptying an array will also update all the reference variables that point to the original array.

For instance:

var arrayList = ['a', 'b', 'c', 'd', 'e', 'f']; // Created array
var anotherArrayList = arrayList;  // Referenced arrayList by another variable
arrayList.length = 0; // Empty the array by setting length to 0
console.log(anotherArrayList); // Output []


Method 3

 
arrayList.splice(0, arrayList.length);

Above implementation will also work perfectly. This way of empty the array will also update all the references of the original array.
var arrayList = ['a', 'b', 'c', 'd', 'e', 'f']; // Created array
var anotherArrayList = arrayList;  // Referenced arrayList by another variable
arrayList.splice(0, arrayList.length); // Empty the array by setting length to 0
console.log(anotherArrayList); // Output []


Method 4

 
while(arrayList.length) {
  arrayList.pop();
}

Above implementation can also empty the array. But not recommended to use often.

Answer:

The best way to find whether an object is instance of a particular class or not using toString method from Object.prototype
 

var arrayList = [1 , 2, 3];

One of the best use cases of type checking of an object is when we do method overloading in JavaScript. To understand this, let's say we have a method called greet which can take a single string and also a list of strings. To make our greet method workable in both situation we need to know what kind of parameter is being passed: is it single value or list of values?
 
function greet(param) {
  if() {
    // here have to check whether param is array or not
  }
  else {
  }
}

However, in the above implementation it might not necessary to check the type of the array, we can check for single value string and put array logic code in else block, let see below code for the same.
 
function greet(param) {
   if(typeof param === 'string') {
   }
   else {
     // If param is of type array then this block of code would execute
   }
 }
 

Now it's fine we can go with the previous two implementations, but when we have a situation like a parameter can be single valuearray, and object type then we will be in trouble.

Coming back to checking the type of an object, As we mentioned that we can use Object.prototype.toString

 

if(Object.prototype.toString.call(arrayList) === '[object Array]') {
  console.log('Array!');
}

If you are using jQuery then you can also used jQuery isArray method:
 
if($.isArray(arrayList)) {
  console.log('Array');
} else {
  console.log('Not an array');
}
 

FYI jQuery uses Object.prototype.toString.call internally to check whether an object is an array or not.

In modern browser, you can also use:

 

Array.isArray(arrayList);

Array.isArray is supported by Chrome 5, Firefox 4.0, IE 9, Opera 10.5 and Safari 5

Answer:

Let's take the following function expression
 

var foo = function foo() {
     return 12;
 }

In JavaScript var-declared variables and functions are hoisted. Let's take function hoisting first. Basically, the JavaScript interpreter looks ahead to find all the variable declaration and hoists them to the top of the function where it's declared. For example:
 
foo(); // Here foo is still undefined
var foo = function foo() {
  return 12;
};

The code above behind the scene look something like this:
 
var foo = undefined;
foo(); // Here foo is undefined
foo = function foo() {
  // Some code stuff
}

var foo = undefined;
foo = function foo() {
  // Some code stuff
}
foo(); // Now foo is defined here

Answer:

typeof is an operator that returns a string with the type of whatever you pass.

The typeof operator checks if a value belongs to one of the seven basic types: numberstringbooleanobjectfunctionundefined or Symbol.

typeof(null) will return object.

instanceof is much more intelligent: it works on the level of prototypes. In particular, it tests to see if the right operand appears anywhere in the prototype chain of the left. instanceof doesn’t work with primitive types. The instanceof operator checks the current object and returns true if the object is of the specified type, for example:

 

var dog = new Animal();
dog instanceof Animal; // Output : true

Here dog instanceof Animal is true since dog inherits from Animal.prototype
 
var name = new String("xyz");
name instanceof String; // Output : true
 

Answer:

If your are familiar with Object-oriented programming, More likely familiar to thinking of functions, methods, and class constructors as three separate things. But In JavaScript, these are just three different usage patterns of one single construct.

functions : The simplest usages of function call:

 

function helloWorld(name) {
  return "hello world, " + name;
}

helloWorld("JS Geeks"); // "hello world JS Geeks"

Methods in JavaScript are nothing more than object properties that are functions.
 
var obj = {
  helloWorld : function() {
    return "hello world, " + this.name;
  },
  name: 'John Carter'
}
obj.helloWorld(); // // "hello world John Carter"

Notice how helloWorld refer to this properties of obj. Here it's clear or you might have already understood that this gets bound to obj. But the interesting point that we can copy a reference to the same function helloWorld in another object and get a difference answer. Let see:
 
var obj2 = {
  helloWorld : obj.helloWorld,
  name: 'John Doe'
}
obj2.helloWorld(); // "hello world John Doe"
 

You might be wonder what exactly happens in a method call here. Here we call the expression itself determine the binding of this this, The expression obj2.helloWorld() looks up the helloWorld property of obj and calls it with receiver object obj2.

The third use of functions is as constructors. Like function and method, constructors are defined with function.

 

function Employee(name, age) {
  this.name = name;
  this.age = age;
}

var emp1 = new Employee('John Doe', 28);
emp1.name; // "John Doe"
emp1.age; // 28
 

Unlike function calls and method calls, a constructor call new Employee('John Doe', 28) creates a brand new object and passes it as the value of this, and implicitly returns the new object as its result.

The primary role of the constructor function is to initialize the object.

Answer:

It’s a technology that allows your web application to use cached resources first, and provide default experience offline, before getting more data from the network later. This principle is commonly known as Offline First.

Service Workers actively use promises. A Service Worker has to be installed,activated and then it can react on fetch, push and sync events.

As of 2017, Service Workers are not supported in IE and Safari.

Answer:

In JS, that difference is quite subtle. A function is a piece of code that is called by name and function itself not associated with any object and not defined inside any object. It can be passed data to operate on (i.e. parameter) and can optionally return data (the return value).
 

/ Function statement
function myFunc() {
  // Do some stuff;
}

// Calling the function
myFunc();
 

Here myFunc() function call is not associated with object hence not invoked through any object.

A function can take a form of immediately invoked function expression (IIFE):

 

// Anonymous Self-invoking Function
(function() {
  // Do some stuff;
})();

Finally there are also arrow functions:
 
const myFunc = arg => {
    console.log("hello", arg)
} 
 

A method is a piece of code that is called by its name and that is associated with the object. Methods are functions. When you call a method like this obj1.myMethod(), the reference to obj1 gets assigned (bound) to this variable. In other words, the value of this will be obj1 inside myMethod.

Here are some examples of methods:
Example 1

 

var obj1 = {
  attribute: "xyz",
  myMethod: function () {  // Method
    console.log(this.attribute);
  }
};

// Call the method
obj1.myMethod();

 Here obj1 is an object and myMethod is a method which is associated with obj1.
In ES6 we have classes. There the methods will look like this:
 
class MyAwesomeClass {
  myMethod() {
    console.log("hi there");
  }
}

const obj1 = new MyAwesomeClass();
obj1.myMethod();

Understand: the method is not some kind of special type of a function, and it's not about how you declare a function. It's the way we call a function. Look at that:
 
ar obj1 = {
  prop1: "buddy"
}; 
var myFunc = function () {
  console.log("Hi there", this);
};
// let's call myFunc as a function: 
myFunc(); // will output "Hi there undefined" or "Hi there Window"
 
obj1.myMethod = myFunc;
//now we're calling myFunc as a method of obj1, so this will point to obj1
obj1.myMethod(); // will print "Hi there" following with obj1. 
 

Answer:

IIFE a function that runs as soon as it's defined. Usually it's anonymous (doesn't have a function name), but it also can be named. Here's an example of IIFE:
 

(function() {
  console.log("Hi, I'm IIFE!");
})();
// outputs "Hi, I'm IIFE!"
 

Explanation :

So, here's how it works. Remember the difference between function statements (function a () {}) and function expressions (var a = function() {})? So, IIFE is a function expression. To make it an expression we surround our function declaration into the parens. We do it to explicitly tell the parser that it's an expression, not a statement (JS doesn't allow statements in parens).

After the function you can see the two () braces, this is how we run the function we just declared.

That's it. The rest is details.

1. The function inside IIFE doesn't have to be anonymous. This one will work perfectly fine and will help to detect your function in a stacktrace during debugging:

 

(function myIIFEFunc() {
  console.log("Hi, I'm IIFE!");
})();
// outputs "Hi, I'm IIFE!"

2. It can take some parameters:
 
(function myIIFEFunc(param1) {
  console.log("Hi, I'm IIFE, " + param1);
})("Yuri");
// outputs "Hi, I'm IIFE, Yuri!"
Here there value "Yuri" is passed to the param1 of the function.

3. It can return a value:
var result = (function myIIFEFunc(param1) {
  console.log("Hi, I'm IIFE, " + param1);
  return 1;
})("Yuri");
// outputs "Hi, I'm IIFE, Yuri!"
// result variable will contain 1
 

3. You don't have to surround the function declaration into parens, although it's the most common way to define IIFE. Instead you can use any of the following forms:

  • ~function(){console.log("hi I'm IIFE")}()
  • !function(){console.log("hi I'm IIFE")}()
  • +function(){console.log("hi I'm IIFE")}()
  • -function(){console.log("hi I'm IIFE")}()
  • (function(){console.log("hi I'm IIFE")}());
  • var i = function(){console.log("hi I'm IIFE")}();
  • true && function(){ console.log("hi I'm IIFE") }();
  • 0, function(){ console.log("hi I'm IIFE") }();
  • new function(){ console.log("hi I'm IIFE") }
  • new function(){ console.log("hi I'm IIFE") }()

Please don't use all these forms to impress colleagues, but be prepared that you can encounter them in someone's code.

Answer:

The singleton pattern is an often used JavaScript design pattern. It provides a way to wrap the code into a logical unit that can be accessed through a single variable. The Singleton design pattern is used when only one instance of an object is needed throughout the lifetime of an application. In JavaScript, Singleton pattern have many uses, they can be used for NameSpacing, which reduce the number of global variables in your page (prevent from polluting global space), organizing the code in a consistent manner, which increase the readability and maintainability of your pages.

There are two important points in the traditional definition of Singleton pattern:

  • There should be only one instance allowed for a class and
  • We should allow global point of access to that single instance

Let me define singleton pattern in JavaScript context:

It is an object that is used to create namespace and group together a related set of methods and attributes (encapsulation) and if we allow to initiate then it can be initiated only once.

In JavaScript, we can create singleton though object literal. However, there is some another way but that I will cover in next post.

A singleton object consists of two parts: The object itself, containing the members (Both methods and attributes) within it, and global variable used to access it. The variable is global so that object can be accessed anywhere in the page, this is a key feature of the singleton pattern.

JavaScript: A Singleton as a Namespace

As I have already stated above that singleton can be used to declare Namespace in JavaScript. NameSpacing is a large part of responsible programming in JavaScript. Because everything can be overwritten, and it is very easy to wipe out variable by mistake or a function, or even a class without even knowing it. A common example which happens frequently when you are working with another team member parallel,

 

function findUserName(id) {

}

/* Later in the page another programmer
added code */
var findUserName = $('#user_list');

/* You are trying to call :( */
console.log(findUserName())

One of the best ways to prevent accidentally overwriting variable is to namespace your code within a singleton object.
 
/*  Using Namespace */

var MyNameSpace = {
  findUserName : function(id) {},
  // Other methods and attribute go here as well
}

/* Later in the page another programmer
added code */
var findUserName = $('#user_list');

/* You are trying to call and you make this time workable */
console.log(MyNameSpace.findUserName());
 

Singleton Design Pattern Implementation

/* Lazy Instantiation skeleton for a singleton pattern */

var MyNameSpace = {};
MyNameSpace.Singleton = (function() {

  // Private attribute that holds the single instance
  var singletonInstance;  

  // All of the normal code goes here
  function constructor() {
    // Private members
    var privateVar1 = "Nishant";
    var privateVar2 = [1,2,3,4,5];

    function privateMethod1() {
      // code stuff
    }

    function privateMethod1() {
      // code stuff
    }

    return {
      attribute1 : "Nishant",
      publicMethod: function() {
        alert("Nishant");// some code logic
      }
    }
  }

  return {
    // public method (Global access point to Singleton object)
    getInstance: function() {
      //instance already exist then return  
      if(!singletonInstance) {
        singletonInstance = constructor();
      }
      return singletonInstance;           
    }           
  }

})();   

// getting access of publicMethod
console.log(MyNamespace.Singleton.getInstance().publicMethod());

The singleton implemented above is easy to understand. The singleton class maintains a static reference to the lone singleton instance and return that reference from the static getInstance() method.
 

Answer:

Method 1: Function based

This method is useful if we want to create several similar objects. In the code sample below, we wrote the function Employee and used it as a constructor by calling it with the new operator.
 
function Employee(fName, lName, age, salary){
  	this.firstName = fName;
  	this.lastName = lName;
  	this.age = age;
  	this.salary = salary;
  }

  // Creating multiple object which have similar property but diff value assigned to object property.
  var employee1 = new Employee('John', 'Moto', 24, '5000$');
  var employee2 = new Employee('Ryan', 'Jor', 26, '3000$');
  var employee3 = new Employee('Andre', 'Salt', 26, '4000$');
 

Method 2: Object Literal

Object Literal is best way to create an object and this is used frequently. Below is code sample for create employee object which contains property as well as method.
 
var employee = {
	name : 'Nishant',
	salary : 245678,
	getName : function(){
		return this.name;
	}
}

The code sample below is Nested Object Literal, Here address is an object inside employee object.
 
var employee = {
	name : 'Nishant',
	salary : 245678,
	address : {
		addressLine1 : 'BITS Pilani',
		addressLine2 : 'Vidya Vihar'.
		phoneNumber: {
		  workPhone: 7098889765,
		  homePhone: 1234567898
		}
	}
}
 

Method 3: From Object using new keyword

In the code below, a sample object has been created using Object's constructor function.
var employee = new Object(); // Created employee object using new keywords and Object()
employee.name = 'Nishant';
employee.getName = function(){
	return this.name;
}
 

Method 4:** Using Object.create

Object.create(obj) will create a new object and set the obj as its prototype. It’s a modern way to create objects that inherit properties from other objects. Object.create function doesn’t run the constructor. You can use Object.create(null) when you don’t want your object to inherit the properties of Object.


 

Answer:

Suppose we have given an object person
 

var person = {
	name: 'Nishant',
	age : 24
}

Here the person object has a name and age property. Now we are trying to access the salary property which we haven't declared on the person object so while accessing it will return undefined. So how we will ensure whether property is undefined or not before performing some operation over it?
 

Explanation:

We can use typeof operator to check undefined
 

if(typeof someProperty === 'undefined'){
	console.log('something is undefined here');
}

Now we are trying to access salary property of person object.
 
if(typeof person.salary === 'undefined'){
	console.log("salary is undefined here because we haven't declared");
}
 

Answer:

We use promises for handling asynchronous interactions in a sequential manner. They are especially useful when we need to do an async operation and THEN do another async operation based on the results of the first one. For example, if you want to request the list of all flights and then for each flight you want to request some details about it. The promise represents the future value. It has an internal state (pendingfulfilled and rejected) and works like a state machine.

A promise object has then method, where you can specify what to do when the promise is fulfilled or rejected.

You can chain then() blocks, thus avoiding the callback hell. You can handle errors in the catch() block. After a promise is set to fulfilled or rejected state, it becomes immutable.

Also mention that you know about more sophisticated concepts:

  • async/await which makes the code appear even more linear
  • RxJS observables can be viewed as the recyclable promises

Be sure that you can implement the promise, read one of the articles on a topic, and learn the source code of the simplest promise implementation.

Answer:

Let say we have person object with property name and age
 

var person = {
	name: 'Nishant',
	age: 24
}
 

Now we want to check whether name property exist in person object or not ?

In JavaScript object can have own property, in above example name and age is own property of person object. Object also have some of inherited property of base object like toString is inherited property of person object.

So how we will check whether property is own property or inherited property.

Method 1: We can use in operator on objet to check own property or inherited property.
 

console.log('name' in person); // checking own property print true 
console.log('salary' in person); // checking undefined property print false
 

in operator also look into inherited property if it doesn't find property defined as own property. For instance If I check existence of toString property as we know that we haven't declared this property on person object so in operator look into there base property.

Here
 

console.log('toString' in person); // Will print true

If we want to test property of object instance not inherited properties then we will use hasOwnProperty method of object instance.
console.log(person.hasOwnProperty('toString')); // print false
console.log(person.hasOwnProperty('name')); // print true
console.log(person.hasOwnProperty('salary')); // print false

 

Answer:

We always encounter in such situation where we need to know whether value is type of array or not.

For instance : the code below perform some operation based value type


function(value){
	if("value is an array"){
		// Then perform some operation
	}else{
		// otherwise
	}
}

Let's discuss some way to detect an array in JavaScript.
 

Method 1:

Juriy Zaytsev (Also known as kangax) proposed an elegant solution to this.

 

function isArray(value){
		return Object.prototype.toString.call(value) === '[object Array]';
	}

This approach is most popular way to detecting a value of type array in JavaScript and recommended to use. This approach relies on the fact that, native toString() method on a given value produce a standard string in all browser.

Method 2:

Duck typing test for array type detection

 

// Duck typing arrays
 function isArray(value){
 	return typeof value.sort === 'function';
 }

As we can see above isArray method will return true if value object have sort method of type function. Now assume you have created a object with sort method
 
var bar = {
		sort: function(){
			// Some code 
		}
	}
 

Now when you check isArray(bar) then it will return true because bar object has sort method, But the fact is bar is not an array.

So this method is not a best way to detect an array as you can see it's not handle the case when some object has sort method.

Method 3:

ECMAScript 5 has introduced Array.isArray() method to detect an array type value. The sole purpose of this method is accurately detecting whether a value is an array or not.

In many JavaScript libraries you may see the code below for detecting an value of type array.

 

function(value){
   // ECMAScript 5 feature
	if(typeof Array.isArray === 'function'){
		return Array.isArray(value);
	}else{
	   return Object.prototype.toString.call(value) === '[object Array]';
	}
}
 

Method 4:

You can query the constructor name:

function isArray(value) {
	return value.constructor.name === "Array";
}
 

Answer:

In Javascript Object are called as reference type, Any value other then primitive is definitely a reference type. There are several built-in reference type such as ObjectArrayFunctionDatenull and Error.

Detecting object using typeof operator

 

console.log(typeof {});           // object
console.log(typeof []);           // object
console.log(typeof new Array());  // object
console.log(typeof null);         // object 
console.log(typeof new RegExp()); // object
console.log(typeof new Date());   // object
 

But the downside of using typeof operator to detect an object is that typeof returns object for null (However this is fact that null is an object in JavaScript).

The best way to detect an object of specific reference type using instanceof operator.

 

//Detecting an array
if(value instanceof Array){
	console.log("value is type of array");
}
 
// Employee constructor function
function Employee(name){
	this.name = name; // Public property
}

var emp1 = new Employee('John');

console.log(emp1 instanceof Employee); // true

instanceof not only check the constructor which is used to create an object but also check it's prototype chain see below example.
 
console.log(emp1 instanceof Object); // true
 

Answer:

The ECMAScript 5 Object.create() method is the easiest way for one object to inherit from another, without invoking a constructor function.
For instance:
 

var employee = {
  name: 'Nishant',
  displayName: function () {
    console.log(this.name);
  }
};

var emp1 = Object.create(employee);
console.log(emp1.displayName());  // output "Nishant"
 

In the example above, we create a new object emp1 that inherits from employee. In other words emp1's prototype is set to employee. After this emp1 is able to access the same properties and method on employee until new properties or method with the same name are defined.

For instance: Defining displayName() method on emp1 will not automatically override the employee displayName.

 

emp1.displayName = function() {
	console.log('xyz-Anonymous');
};

employee.displayName(); //Nishant
emp1.displayName();//xyz-Anonymous

In addition to this Object.create() method also allows to specify a second argument which is an object containing additional properties and methods to add to the new object.
 
var emp1 = Object.create(employee, {
	name: {
		value: "John"
	}
});

emp1.displayName(); // "John"
employee.displayName(); // "Nishant"
 

In the example above, emp1 is created with it's own value for name, so calling displayName() method will display "John" instead of "Nishant".

Object created in this manner give you full control over newly created object. You are free to add, remove any properties and method you want.

Answer:

et say we have Person class which has name, age, salary properties and incrementSalary() method.
 

function Person(name, age, salary) {
  this.name = name;
  this.age = age;
  this.salary = salary;
  this.incrementSalary = function (byValue) {
    this.salary = this.salary + byValue;
  };
}

Now we wish to create Employee class which contains all the properties of Person class and wanted to add some additional properties into Employee class.
 
function Employee(company){
	this.company = company;
}

//Prototypal Inheritance 
Employee.prototype = new Person("Nishant", 24,5000);

In the example above, Employee type inherits from Person. It does so by assigning a new instance of Person to Employee prototype. After that, every instance of Employee inherits its properties and methods from Person.
 
//Prototypal Inheritance 
Employee.prototype = new Person("Nishant", 24,5000);

var emp1 = new Employee("Google");

console.log(emp1 instanceof Person); // true
console.log(emp1 instanceof Employee); // true

Let's understand Constructor inheritance
 
/Defined Person class
function Person(name){
	this.name = name || "Nishant";
}

var obj = {};

// obj inherit Person class properties and method 
Person.call(obj); // constructor inheritance

console.log(obj); // Object {name: "Nishant"}

Here we saw calling Person.call(obj) define the name properties from Person to obj.
 
console.log(name in obj); // true

Type-based inheritance is best used with developer defined constructor function rather than natively in JavaScript. In addition to this also allows flexibility in how we create similar type of object.

Answer:

ECMAScript 5 introduce several methods to prevent modification of object which lock down object to ensure that no one, accidentally or otherwise, change functionality of Object.

There are three levels of preventing modification:

1: Prevent extensions :

No new properties or methods can be added to the object, but one can change the existing properties and method.

For example:

 

var employee = {
	name: "Nishant"
};

// lock the object 
Object.preventExtensions(employee);

// Now try to change the employee object property name
employee.name = "John"; // work fine 

//Now try to add some new property to the object
employee.age = 24; // fails silently unless it's inside the strict mode
 

2: Seal :

It is same as prevent extension, in addition to this also prevent existing properties and methods from being deleted.

To seal an object, we use Object.seal() method. you can check whether an object is sealed or not using Object.isSealed();

var employee = {
	name: "Nishant"
};

// Seal the object 
Object.seal(employee);

console.log(Object.isExtensible(employee)); // false
console.log(Object.isSealed(employee)); // true

delete employee.name // fails silently unless it's in strict mode

// Trying to add new property will give an error
employee.age = 30; // fails silently unless in strict mode
 

when an object is sealed, its existing properties and methods can't be removed. Sealed object are also non-extensible.

3: Freeze :

Same as seal, In addition to this prevent existing properties methods from being modified (All properties and methods are read only).

To freeze an object, use Object.freeze() method. We can also determine whether an object is frozen using Object.isFrozen();

 

var employee = {
	name: "Nishant"
};

//Freeze the object
Object.freeze(employee); 

// Seal the object 
Object.seal(employee);

console.log(Object.isExtensible(employee)); // false
console.log(Object.isSealed(employee));     // true
console.log(Object.isFrozen(employee));     // true


employee.name = "xyz"; // fails silently unless in strict mode
employee.age = 30;     // fails silently unless in strict mode
delete employee.name   // fails silently unless it's in strict mode
 

Frozen objects are considered both non-extensible and sealed.

Recommended:

If you are decided to prevent modification, sealed, freeze the object then use in strict mode so that you can catch the error.

For example:

 

"use strict";

var employee = {
	name: "Nishant"
};

//Freeze the object
Object.freeze(employee); 

// Seal the object 
Object.seal(employee);

console.log(Object.isExtensible(employee)); // false
console.log(Object.isSealed(employee));     // true
console.log(Object.isFrozen(employee));     // true


employee.name = "xyz"; // fails silently unless in strict mode
employee.age = 30;     // fails silently unless in strict mode
delete employee.name;  // fails silently unless it's in strict mode
 

Answer:

If you are coming from python/c# you might be using default value for function parameter incase value(formal parameter) has not been passed. For instance :
 

// Define sentEmail function 
// configuration : Configuration object
// provider : Email Service provider, Default would be gmail
def sentEmail(configuration, provider = 'Gmail'):
	# Your code logic
 

In Pre ES6/ES2015

There are a lot of ways by which you can achieve this in pre ES2015.

Let's understand the code below by which we achieved setting default parameter value.

Method 1: Setting default parameter value

function sentEmail(configuration, provider) {
  // Set default value if user has not passed value for provider
  provider = typeof provider !== 'undefined' ? provider : 'Gmail'  
  // Your code logic
;
}
// In this call we are not passing provider parameter value
sentEmail({
  from: 'xyz@gmail.com',
  subject: 'Test Email'
});
// Here we are passing Yahoo Mail as a provider value
sentEmail({
  from: 'xyz@gmail.com',
  subject: 'Test Email'
}, 'Yahoo Mail');

Method 2: Setting default parameter value
 
unction sentEmail(configuration, provider) {
  // Set default value if user has not passed value for provider
  provider = provider || 'Gmail'  
  // Your code logic
;
}
// In this call we are not passing provider parameter value
sentEmail({
  from: 'xyz@gmail.com',
  subject: 'Test Email'
});
// Here we are passing Yahoo Mail as a provider value
sentEmail({
  from: 'xyz@gmail.com',
  subject: 'Test Email'
}, 'Yahoo Mail');

Method 3: Setting default parameter value in ES6
 
function sendEmail(configuration, provider = "Gmail") {
  // Set default value if user has not passed value for provider
  
  // Value of provider can be accessed directly
  console.log(`Provider: ${provider}`);
}

// In this call we are not passing provider parameter value
sentEmail({
  from: 'xyz@gmail.com',
  subject: 'Test Email'
});
// Here we are passing Yahoo Mail as a provider value
sentEmail({
  from: 'xyz@gmail.com',
  subject: 'Test Email'
}, 'Yahoo Mail');
 

Answer:

Object can have properties that don't show up when you iterate through object using for...in loop or using Object.keys() to get an array of property names. This properties is know as non-enumerable properties.

Let say we have following object

 

var person = {
	name: 'John'
};
person.salary = '10000$';
person['country'] = 'USA';

console.log(Object.keys(person)); // ['name', 'salary', 'country']
 

As we know that person object properties namesalary ,country are enumerable hence it's shown up when we called Object.keys(person).

To create a non-enumerable property we have to use Object.defineProperty(). This is a special method for creating non-enumerable property in JavaScript.

 

var person = {
	name: 'John'
};
person.salary = '10000$';
person['country'] = 'USA';

// Create non-enumerable property
Object.defineProperty(person, 'phoneNo',{
	value : '8888888888',
	enumerable: false
})

Object.keys(person); // ['name', 'salary', 'country']
 

In the example above phoneNo property didn't show up because we made it non-enumerable by setting enumerable:false

Bonus

Now let's try to change value of phoneNo

 

person.phoneNo = '7777777777'; 

Object.defineProperty() also lets you create read-only properties as we saw above, we are not able to modify phoneNo value of a person object. This is because descriptor has writable property, which is false by default. Changing non-writable property value will return error in strict mode. In non-strict mode it won't through any error but it won't change the value of phoneNo.

Answer:

Function binding falls in advance JavaScript category and this is very popular technique to use in conjunction with event handler and callback function to preserve code execution context while passing function as a parameter.

Let's consider the following example:

 

ar clickHandler = {
	message: 'click event handler',
	handleClick: function(event) {
		console.log(this.message);
	}
};

var btn = document.getElementById('myBtn');
// Add click event to btn
btn.addEventListener('click', clickHandler.handleClick);
 

Here in this example clickHandler object is created which contain message properties and handleClick method.

We have assigned handleClick method to a DOM button, which will be executed in response of click. When the button is clicked, then handleClick method is being called and console message. Here console.log should log the click event handler message but it actually log undefined.

The problem of displaying undefined is because of the execution context of clickHandler.handleClick method is not being saved hence this pointing to button btn object. We can fix this issue using bind method.

 

var clickHandler = {
	message: 'click event handler',
	handleClick: function(event) {
		console.log(this.message);
	}
};

var btn = document.getElementById('myBtn');
// Add click event to btn and bind the clickHandler object
btn.addEventListener('click', clickHandler.handleClick.bind(clickHandler));

bind method is available to all the function similar to call and apply method which take argument value of this.

Answer:

Following are the advantages of using JavaScript −

  • Less server interaction − You can validate user input before sending the page off to the server. This saves server traffic, which means less load on your server.

  • Immediate feedback to the visitors − They don't have to wait for a page reload to see if they have forgotten to enter something.

  • Increased interactivity − You can create interfaces that react when the user hovers over them with a mouse or activates them via the keyboard.

  • Richer interfaces − You can use JavaScript to include such items as drag-and-drop components and sliders to give a Rich Interface to your site visitors.

Answer:

We can not treat JavaScript as a full fledged programming language. It lacks the following important features −

  • Client-side JavaScript does not allow the reading or writing of files. This has been kept for security reason.

  • JavaScript can not be used for Networking applications because there is no such support available.

  • JavaScript doesn't have any multithreading or multiprocess capabilities.

Answer:

Yes! JavaScript is a case-sensitive language. This means that language keywords, variables, function names, and any other identifiers must always be typed with a consistent capitalization of letters.

Answer:

You can write and read properties of an object using the dot notation as follows −
 

// Getting object properties
emp.name  // ==> Zara
emp.age   // ==> 10
// Setting object properties
emp.name = "Daisy"  // <== Daisy
emp.age  =  20      // <== 20
 

Answer:

You can define arrays using the array literal as follows −
 

var x = [];
var y = [1, 2, 3, 4, 5];
 

Answer:

An array has a length property that is useful for iteration. We can read elements of an array as follows −
 

var x = [1, 2, 3, 4, 5];
for (var i = 0; i < x.length; i++) {
   // Do something with x[i]
}
 

Answer:

A named function has a name when it is defined. A named function can be defined using function keyword as follows −
 

function named(){
   // do some stuff here
}
 

Answer:

A function in JavaScript can be either named or anonymous.

Answer:

An anonymous function can be defined in similar way as a normal function but it would not have any name.

Answer:

Yes! An anonymous function can be assigned to a variable.

Answer:

Yes! An anonymous function can be passed as an argument to another function.

Answer:

JavaScript variable arguments represents the arguments passed to a function.

Answer:

Using typeof operator, we can get the type of arguments passed to a function. For example −
 

function func(x){
   console.log(typeof x, arguments.length);
}
func();                //==> "undefined", 0
func(1);               //==> "number", 1
func("1", "2", "3");   //==> "string", 3
 

Answer:

Using arguments.length property, we can get the total number of arguments passed to a function. For example −
 

function func(x){
   console.log(typeof x, arguments.length);
}
func();                //==> "undefined", 0
func(1);               //==> "number", 1
func("1", "2", "3");   //==> "string", 3
 

Answer:

The arguments object has a callee property, which refers to the function you're inside of. For example −
 

function func() {
   return arguments.callee; 
}
func();    
 

Answer:

JavaScript famous keyword this always refers to the current context.

Answer:

The scope of a variable is the region of your program in which it is defined. JavaScript variable will have only two scopes.

  • Global Variables − A global variable has global scope which means it is visible everywhere in your JavaScript code.

  • Local Variables − A local variable will be visible only within a function where it is defined. Function parameters are always local to that function.

Answer:

A local variable takes precedence over a global variable with the same name.

Answer:

A callback is a plain JavaScript function passed to some method as an argument or option. Some callbacks are just events, called to give the user a chance to react when a certain state is triggered.

Answer:

charAt() method returns the character at the specified index.

Answer:

concat() method returns the character at the specified index.

Answer:

forEach() method calls a function for each element in the array.

Answer:

indexOf() method returns the index within the calling String object of the first occurrence of the specified value, or −1 if not found.

Answer:

length() method returns the length of the string.

Answer:

pop() method removes the last element from an array and returns that element.

Answer:

push() method adds one or more elements to the end of an array and returns the new length of the array.

Answer:

reverse() method reverses the order of the elements of an array −− the first becomes the last, and the last becomes the first.

Answer:

sort() method sorts the elements of an array.

Answer:

substr() method returns the characters in a string beginning at the specified location through the specified number of characters.

Answer:

toLowerCase() method returns the calling string value converted to lower case.

Answer:

toUpperCase() method returns the calling string value converted to upper case.

Answer:

toString() method returns the string representation of the number's value.

Answer:

While naming your variables in JavaScript keep following rules in mind.

You should not use any of the JavaScript reserved keyword as variable name. These keywords are mentioned in the next section. For example, break or boolean variable names are not valid.

JavaScript variable names should not start with a numeral (0-9). They must begin with a letter or the underscore character. For example, 123test is an invalid variable name but _123test is a valid one.

JavaScript variable names are case sensitive. For example, Name and name are two different variables.

Answer:

The typeof is a unary operator that is placed before its single operand, which can be of any type. Its value is a string indicating the data type of the operand.

The typeof operator evaluates to "number", "string", or "boolean" if its operand is a number, string, or boolean value and returns true or false based on the evaluation.

Answer:

It returns "object".

Answer:

JavaScript can also manipulate cookies using the cookie property of the Document object. JavaScript can read, create, modify, and delete the cookie or cookies that apply to the current web page.

Answer:

The simplest way to create a cookie is to assign a string value to the document.cookie object, which looks like this −

Syntax −

 

document.cookie = "key1 = value1; key2 = value2; expires = date";

Here expires attribute is option. If you provide this attribute with a valid date or time then cookie will expire at the given date or time and after that cookies' value will not be accessible.

Answer:

Reading a cookie is just as simple as writing one, because the value of the document.cookie object is the cookie. So you can use this string whenever you want to access the cookie.

The document.cookie string will keep a list of name = value pairs separated by semicolons, where name is the name of a cookie and value is its string value.

You can use strings' split() function to break the string into key and values.

Answer:

Sometimes you will want to delete a cookie so that subsequent attempts to read the cookie return nothing. To do this, you just need to set the expiration date to a time in the past.

Answer:

this is very simple to do a page redirect using JavaScript at client side. To redirect your site visitors to a new page, you just need to add a line in your head section as follows −
 

//window.location="http://www.newlocation.com";

Answer:

JavaScript helps you to implement this functionality using print function of window object. The JavaScript print function window.print() will print the current web page when executed.

Answer:

The Date object is a datatype built into the JavaScript language. Date objects are created with the new Date( ).

Once a Date object is created, a number of methods allow you to operate on it. Most methods simply allow you to get and set the year, month, day, hour, minute, second, and millisecond fields of the object, using either local time or UTC (universal, or GMT) time.

Answer:

The Number object represents numerical date, either integers or floating-point numbers. In general, you do not need to worry about Number objects because the browser automatically converts number literals to instances of the number class.

Syntax −

Creating a number object −

 

var val = new Number(number);

If the argument cannot be converted into a number, it returns NaN (Not-a-Number).

Answer:

The latest versions of JavaScript added exception handling capabilities. JavaScript implements the try...catch...finally construct as well as the throw operator to handle exceptions.

You can catch programmer-generated and runtime exceptions, but you cannot catch JavaScript syntax errors.

Answer:

The onerror event handler was the first feature to facilitate error handling for JavaScript. The error event is fired on the window object whenever an exception occurs on the page.

The onerror event handler provides three pieces of information to identify the exact nature of the error −

  • Error message − The same message that the browser would display for the given error.

  • URL − The file in which the error occurred.

  • Line number − The line number in the given URL that caused the error.

Answer:

There are many ways to create objects in javascript as below
1. Object constructor:
The simplest way to create an empty object is using the Object constructor. Currently this approach is not recommended.
 

var object = new Object();

2. Object's create method:
The create method of Object creates a new object by passing the prototype object as a parameter
 
var object = Object.create(null);

3. Object literal syntax:
The object literal syntax is equivalent to create method when it passes null as parameter
 
var object = {};

4. Function constructor:
Create any function and apply the new operator to create object instances,
 
function Person(name){
   var object = {};
   object.name=name;
   object.age=21;
   return object;
}
var object = new Person("Sudheer");

5. Function constructor with prototype:
This is similar to function constructor but it uses prototype for their properties and methods,
 
function Person(){}
Person.prototype.name = "Sudheer";
var object = new Person();

This is equivalent to an instance created with an object create method with a function prototype and then call that function with an instance and parameters as arguments.
 
function func {};

new func(x, y, z);

(OR)
 
// Create a new instance using function prototype.
var newInstance = Object.create(func.prototype)

// Call the function
var result = func.call(newInstance, x, y, z),

// If the result is a non-null object then use it otherwise just use the new instance.
console.log(result && typeof result === 'object' ? result : newInstance);

6. ES6 Class syntax:
ES6 introduces class feature to create the objects
 
class Person {
   constructor(name) {
      this.name = name;
   }
}

var object = new Person("Sudheer");

7. Singleton pattern:
A Singleton is an object which can only be instantiated one time. Repeated calls to its constructor return the same instance and this way one can ensure that they don't accidentally create multiple instances.
 
var object = new function(){
   this.name = "Sudheer";
}
 

Answer:

Prototype chaining is used to build new types of objects based on existing ones. It is similar to inheritance in a class based language. The prototype on object instance is available through Object.getPrototypeOf(object) or proto property whereas prototype on constructors function is available through object.prototype.
 

Answer:

The difference between Call, Apply and Bind can be explained with below examples,

Call: The call() method invokes a function with a given this value and arguments provided one by one

 

var employee1 = {firstName: 'John', lastName: 'Rodson'};
var employee2 = {firstName: 'Jimmy', lastName: 'Baily'};

function invite(greeting1, greeting2) {
    console.log(greeting1 + ' ' + this.firstName + ' ' + this.lastName+ ', '+ greeting2);
}

invite.call(employee1, 'Hello', 'How are you?'); // Hello John Rodson, How are you?
invite.call(employee2, 'Hello', 'How are you?'); // Hello Jimmy Baily, How are you?

Apply: Invokes the function and allows you to pass in arguments as an array
 
var employee1 = {firstName: 'John', lastName: 'Rodson'};
var employee2 = {firstName: 'Jimmy', lastName: 'Baily'};

function invite(greeting1, greeting2) {
    console.log(greeting1 + ' ' + this.firstName + ' ' + this.lastName+ ', '+ greeting2);
}

invite.apply(employee1, ['Hello', 'How are you?']); // Hello John Rodson, How are you?
invite.apply(employee2, ['Hello', 'How are you?']); // Hello Jimmy Baily, How are you?

bind: returns a new function, allowing you to pass in an array and any number of arguments
 
var employee1 = {firstName: 'John', lastName: 'Rodson'};
var employee2 = {firstName: 'Jimmy', lastName: 'Baily'};

function invite(greeting1, greeting2) {
    console.log(greeting1 + ' ' + this.firstName + ' ' + this.lastName+ ', '+ greeting2);
}

var inviteEmployee1 = invite.bind(employee1);
var inviteEmployee2 = invite.bind(employee2);
inviteEmployee1('Hello', 'How are you?'); // Hello John Rodson, How are you?
inviteEmployee2('Hello', 'How are you?'); // Hello Jimmy Baily, How are you?

Call and apply are pretty interchangeable. Both execute the current function immediately. You need to decide whether it’s easier to send in an array or a comma separated list of arguments. You can remember by treating Call is for comma (separated list) and Apply is for Array. Whereas Bind creates a new function that will have this set to the first parameter passed to bind().

Answer:

JSON is a text-based data format following JavaScript object syntax, which was popularized by Douglas Crockford. It is useful when you want to transmit data across a network and it is basically just a text file with an extension of .json, and a MIME type of application/json Parsing: Converting a string to a native object
 

JSON.parse(text)

Stringification: **converting a native object to a string so it can be transmitted across the network
 
JSON.stringify(object)
 

Answer:

The slice() method returns the selected elements in an array as a new array object. It selects the elements starting at the given start argument, and ends at the given optional end argument without including the last element. If you omit the second argument then it selects till the end. Some of the examples of this method are,
 

let arrayIntegers = [1, 2, 3, 4, 5];
let arrayIntegers1 = arrayIntegers.slice(0,2); // returns [1,2]
let arrayIntegers2 = arrayIntegers.slice(2,3); // returns [3]
let arrayIntegers3 = arrayIntegers.slice(4); //returns [5]

Note: Slice method won't mutate the original array but it returns the subset as a new array.

Answer:

The splice() method is used either adds/removes items to/from an array, and then returns the removed item. The first argument specifies the array position for insertion or deletion whereas the option second argument indicates the number of elements to be deleted. Each additional argument is added to the array. Some of the examples of this method are,
 

let arrayIntegersOriginal1 = [1, 2, 3, 4, 5];
let arrayIntegersOriginal2 = [1, 2, 3, 4, 5];
let arrayIntegersOriginal3 = [1, 2, 3, 4, 5];

let arrayIntegers1 = arrayIntegersOriginal1.splice(0,2); // returns [1, 2]; original array: [3, 4, 5]
let arrayIntegers2 = arrayIntegersOriginal2.splice(3); // returns [4, 5]; original array: [1, 2, 3]
let arrayIntegers3 = arrayIntegersOriginal3.splice(3, 1, "a", "b", "c"); //returns [4]; original array: [1, 2, 3, "a", "b", "c", 5]

Note: Splice method modifies the original array and returns the deleted array.

Answer:

Some of the major difference in a tabular form

Slice Splice
Doesn't modify the original array(immutable) Modifies the original array(mutable)
Returns the subset of original array Returns the deleted elements as array
Used to pick the elements from array Used to insert or delete elements to/from array

Answer:

Objects are similar to Maps in that both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Due to this reason, Objects have been used as Maps historically. But there are important differences that make using a Map preferable in certain cases.

  1. The keys of an Object are Strings and Symbols, whereas they can be any value for a Map, including functions, objects, and any primitive.
  2. The keys in Map are ordered while keys added to Object are not. Thus, when iterating over it, a Map object returns keys in order of insertion.
  3. You can get the size of a Map easily with the size property, while the number of properties in an Object must be determined manually.
  4. A Map is an iterable and can thus be directly iterated, whereas iterating over an Object requires obtaining its keys in some fashion and iterating over them.
  5. An Object has a prototype, so there are default keys in the map that could collide with your keys if you're not careful. As of ES5 this can be bypassed by using map = Object.create(null), but this is seldom done.
  6. A Map may perform better in scenarios involving frequent addition and removal of key pairs.

Answer:

JavaScript provides both strict(===, !==) and type-converting(==, !=) equality comparison. The strict operators take type of variable in consideration, while non-strict operators make type correction/conversion based upon values of variables. The strict operators follow the below conditions for different types,

  1. Two strings are strictly equal when they have the same sequence of characters, same length, and same characters in corresponding positions.
  2. Two numbers are strictly equal when they are numerically equal. i.e, Having the same number value. There are two special cases in this,
    1. NaN is not equal to anything, including NaN.
    2. Positive and negative zeros are equal to one another.
  3. Two Boolean operands are strictly equal if both are true or both are false.
  4. Two objects are strictly equal if they refer to the same Object.
  5. Null and Undefined types are not equal with ===, but equal with ==. i.e, null===undefined --> false but null==undefined --> true

Some of the example which covers the above cases,

 

0 == false   // true
0 === false  // false
1 == "1"     // true
1 === "1"    // false
null == undefined // true
null === undefined // false
'0' == false // true
'0' === false // false
[]==[] or []===[] //false, refer different objects in memory
{}=={} or {}==={} //false, refer different objects in memory

Answer:

An arrow function is a shorter syntax for a function expression and does not have its own this, arguments, super, or new.target. These functions are best suited for non-method functions, and they cannot be used as constructors.

Answer:

In Javascript, functions are first class objects. First-class functions means when functions in that language are treated like any other variable.

For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable. For example, in the below example, handler functions assigned to a listener

 

const handler = () => console.log ('This is a click handler function');
document.addEventListener ('click', handler);

Answer:

First-order function is a function that doesn’t accept another function as an argument and doesn’t return a function as its return value.
 

const firstOrder = () => console.log ('I am a first order function!');
 

Answer:

Higher-order function is a function that accepts another function as an argument or returns a function as a return value.
 

const firstOrderFunc = () => console.log ('Hello I am a First order function');
const higherOrder = ReturnFirstOrderFunc => ReturnFirstOrderFunc ();
higherOrder (firstOrderFunc);
 

Answer:

Unary function (i.e. monadic) is a function that accepts exactly one argument. Let us take an example of unary function. It stands for a single argument accepted by a function.
 

const unaryFunction = a => console.log (a + 10); // Add 10 to the given argument and display the value
 

Answer:

Currying is the process of taking a function with multiple arguments and turning it into a sequence of functions each with only a single argument. Currying is named after a mathematician Haskell Curry. By applying currying, a n-ary function turns it into a unary function. Let's take an example of n-ary function and how it turns into a currying function
 

const multiArgFunction = (a, b, c) => a + b + c;
const curryUnaryFunction = a => b => c => a + b + c;
curryUnaryFunction (1); // returns a function: b => c =>  1 + b + c
curryUnaryFunction (1) (2); // returns a function: c => 3 + c
curryUnaryFunction (1) (2) (3); // returns the number 6

Curried functions are great to improve code reusability and functional composition.

Answer:

Pure function is a function where the return value is only determined by its arguments without any side effects. i.e, If you call a function with the same arguments 'n' number of times and 'n' number of places in the application then it will always return the same value. Let's take an example to see the difference between pure and impure functions,
 

//Impure
let numberArray = [];
const impureAddNumber = number => numberArray.push (number);
//Pure
const pureAddNumber = number => argNumberArray =>
  argNumberArray.concat ([number]);

//Display the results
console.log (impureAddNumber (6)); // returns 1
console.log (numberArray); // returns [6]
console.log (pureAddNumber (7) (numberArray)); // returns [6, 7]
console.log (numberArray); // returns [6]

As per above code snippets, Push function is impure itself by altering the array and returning an push number index which is independent of parameter value. Whereas Concat on the other hand takes the array and concatenates it with the other array producing a whole new array without side effects. Also, the return value is a concatenation of the previous array. Remember that Pure functions are important as they simplify unit testing without any side effects and no need for dependency injection. They also avoid tight coupling and make it harder to break your application by not having any side effects. These principles are coming together with Immutability concept of ES6 by giving preference to const over let usage.

Answer:

The let statement declares a block scope local variable. Hence the variables defined with let keyword are limited in scope to the block, statement, or expression on which it is used. Whereas variables declared with the var keyword used to define a variable globally, or locally to an entire function regardless of block scope. Let's take an example to demonstrate the usage,
 

let counter = 30;
if (counter === 30) {
  let counter = 31;
  console.log(counter); // 31
}
console.log(counter); // 30 (because if block variable won't exist here)
 

Answer:

You can list out the differences in a tabular format
 

var let
It is been available from the beginning of JavaScript Introduced as part of ES6
It has function scope It has block scope
Variables will be hoisted Hoisted but not initialized

Let's take an example to see the difference,
 
function userDetails(username) {
   if(username) {
     console.log(salary); // undefined(due to hoisting)
     console.log(age); // error: age is not defined
     let age = 30;
     var salary = 10000;
   }
   console.log(salary); //10000 (accessible to due function scope)
   console.log(age); //error: age is not defined(due to block scope)
}
 

Answer:

Let is a mathematical statement that was adopted by early programming languages like Scheme and Basic. It has been borrowed from dozens of other languages that use let already as a traditional keyword as close to var as possible.

Answer:

If you try to redeclare variables in a switch block then it will cause errors because there is only one block. For example, the below code block throws a syntax error as below,
 

let counter = 1;
switch(x) {
  case 0:
    let name;
    break;

  case 1:
    let name; // SyntaxError for redeclaration.
    break;
}

To avoid this error, you can create a nested block inside a case clause and create a new block scoped lexical environment.
 
let counter = 1;
    switch(x) {
      case 0: {
        let name;
        break;
      }
      case 1: {
        let name; // No SyntaxError for redeclaration.
        break;
      }
    }
 

Answer:

There are a lot of benefits to using modules in favour of a sprawling. Some of the benefits are,

  1. Maintainability
  2. Reusability
  3. Namespacing

Answer:

Memoization is a programming technique which attempts to increase a function’s performance by caching its previously computed results. Each time a memoized function is called, its parameters are used to index the cache. If the data is present, then it can be returned, without executing the entire function. Otherwise the function is executed and then the result is added to the cache. Let's take an example of adding function with memoization,
 

const memoizAddition = () => {
  let cache = {};
 return (value) => {
  if (value in cache) {
   console.log('Fetching from cache');
   return cache[value]; // Here, cache.value cannot be used as property name starts with the number which is not a valid JavaScript  identifier. Hence, can only be accessed using the square bracket notation.
  }
  else {
   console.log('Calculating result');
   let result = value + 20;
   cache[value] = result;
   return result;
  }
 }
}
// returned function from memoizAddition
const addition = memoizAddition();
console.log(addition(20)); //output: 40 calculated
console.log(addition(20)); //output: 40 cached
 

Answer:

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution. Remember that JavaScript only hoists declarations, not initialisation. Let's take a simple example of variable hoisting,
 

console.log(message); //output : undefined
var message = 'The variable Has been hoisted';

The above code looks like as below to the interpreter,
 
var message;
console.log(message);
message = 'The variable Has been hoisted';
 

Answer:

In ES6, Javascript classes are primarily syntactic sugar over JavaScript’s existing prototype-based inheritance. For example, the prototype based inheritance written in function expression as below,
 

function Bike(model,color) {
    this.model = model;
    this.color = color;
}

Bike.prototype.getDetails = function() {
    return this.model + ' bike has' + this.color + ' color';
};

Whereas ES6 classes can be defined as an alternative
 
class Bike{
  constructor(color, model) {
    this.color= color;
    this.model= model;
  }

  getDetails() {
    return this.model + ' bike has' + this.color + ' color';
  }
}
 

Answer:

IndexedDB is a low-level API for client-side storage of larger amounts of structured data, including files/blobs. This API uses indexes to enable high-performance searches of this data.

Answer:

Web storage is an API that provides a mechanism by which browsers can store key/value pairs locally within the user's browser, in a much more intuitive fashion than using cookies. The web storage provides two mechanisms for storing data on the client.

  1. Local storage: It stores data for current origin with no expiration date.
  2. Session storage: It stores data for one session and the data is lost when the browser tab is closed.

Answer:

Post message is a method that enables cross-origin communication between Window objects.(i.e, between a page and a pop-up that it spawned, or between a page and an iframe embedded within it). Generally, scripts on different pages are allowed to access each other if and only if the pages follow same-origin policy(i.e, pages share the same protocol, port number, and host).

Answer:

A cookie is a piece of data that is stored on your computer to be accessed by your browser. Cookies are saved as key/value pairs.
 

document.cookie = "username=John";
 

Answer:

Cookies are used to remember information about the user profile(such as username). It basically involves two steps,

  1. When a user visits a web page, the user profile can be stored in a cookie.
  2. Next time the user visits the page, the cookie remembers the user profile.

Answer:

There are few below options available for a cookie

1. By default, the cookie is deleted when the browser is closed but you can change this behavior by setting expiry date (in UTC time).
 

document.cookie = "username=John; expires=Sat, 8 Jun 2019 12:00:00 UTC";

2. By default, the cookie belongs to a current page. But you can tell the browser what path the cookie belongs to using a path parameter.
 
document.cookie = "username=John; path=/services";
 

Answer:

You can delete a cookie by setting the expiry date as a passed date. You don't need to specify a cookie value in this case. For example, you can delete a username cookie in the current page as below.
 

document.cookie = "username=; expires=Fri, 07 Jun 2019 00:00:00 UTC; path=/;";

Note: You should define the cookie path option to ensure that you delete the right cookie. Some browsers doesn't allow to delete a cookie unless you specify a path parameter.

Answer:

Below are some of the differences between cookie, local storage and session storage,

Feature Cookie Local Storage Session Storage
Accessed on client or server side Both server-side & client-side client-side only client-side only
Lifetime As configured using Expires option until deleted until tab is closed
SSL support Supported
Not supported
Not supported
Maximum data size 4KB 5 MB 5 MB

Answer:

LocalStorage is the same as SessionStorage but it persists the data even when the browser is closed and reopened(i.e it has no expiration time) whereas in sessionStorage data gets cleared when the page session ends.

Answer:

The Window object implements the WindowLocalStorage and WindowSessionStorage objects which has localStorage(window.localStorage) and sessionStorage(window.sessionStorage) properties respectively. These properties create an instance of the Storage object, through which data items can be set, retrieved and removed for a specific domain and storage type (session or local). For example, you can read and write on local storage objects as below
 

localStorage.setItem('logo', document.getElementById('logo').value);
localStorage.getItem('logo');
 

Answer:

The session storage provided methods for reading, writing and clearing the session data
 

// Save data to sessionStorage
sessionStorage.setItem('key', 'value');

// Get saved data from sessionStorage
let data = sessionStorage.getItem('key');

// Remove saved data from sessionStorage
sessionStorage.removeItem('key');

// Remove all saved data from sessionStorage
sessionStorage.clear();
 

Answer:

The StorageEvent is an event that fires when a storage area has been changed in the context of another document. Whereas onstorage property is an EventHandler for processing storage events. The syntax would be as below
 

window.onstorage = functionRef;

Let's take the example usage of onstorage event handler which logs the storage key and it's values
 
window.onstorage = function(e) {
  console.log('The ' + e.key +
    ' key has been changed from ' + e.oldValue +
    ' to ' + e.newValue + '.');
};
 

Answer:

Web storage is more secure, and large amounts of data can be stored locally, without affecting website performance. Also, the information is never transferred to the server. Hence this is a more recommended approach than Cookies.

Answer:

You need to check browser support for localStorage and sessionStorage before using web storage,
 

if (typeof(Storage) !== "undefined") {
  // Code for localStorage/sessionStorage.
} else {
  // Sorry! No Web Storage support..
}
 

Answer:

You need to check browser support for web workers before using it
 

if (typeof(Worker) !== "undefined") {
  // code for Web worker support.
} else {
  // Sorry! No Web Worker support..
}
 

Answer:

You need to follow below steps to start using web workers for counting example

Create a Web Worker File: You need to write a script to increment the count value. Let's name it as counter.js
 

let i = 0;

function timedCount() {
  i = i + 1;
  postMessage(i);
  setTimeout("timedCount()",500);
}

timedCount();

Here postMessage() method is used to post a message back to the HTML page
 1. Create a Web Worker Object: You can create a web worker object by checking for browser support. Let's name this file as web_worker_example.js
 
if (typeof(w) == "undefined") {
  w = new Worker("counter.js");
}

and we can receive messages from web worker
 
w.onmessage = function(event){
  document.getElementById("message").innerHTML = event.data;
};

1. Terminate a Web Worker: Web workers will continue to listen for messages (even after the external script is finished) until it is terminated. You can use the terminate() method to terminate listening to the messages.
 
w.terminate();

Reuse the Web Worker: If you set the worker variable to undefined you can reuse the code
 
w = undefined;
 

Answer:

WebWorkers don't have access to below javascript objects since they are defined in an external files

  1. Window object
  2. Document object
  3. Parent object

Answer:

A callback function is a function passed into another function as an argument. This function is invoked inside the outer function to complete an action. Let's take a simple example of how to use callback function
 

function callbackFunction(name) {
  console.log('Hello ' + name);
}

function outerFunction(callback) {
  let name = prompt('Please enter your name.');
  callback(name);
}

outerFunction(callbackFunction);
 

Answer:

The callbacks are needed because javascript is an event driven language. That means instead of waiting for a response javascript will keep executing while listening for other events. Let's take an example with the first function invoking an API call(simulated by setTimeout) and the next function which logs the message.
 

function firstFunction(){
  // Simulate a code delay
  setTimeout( function(){
    console.log('First function called');
  }, 1000 );
}
function secondFunction(){
  console.log('Second function called');
}
firstFunction();
secondFunction();

Output
// Second function called
// First function called

As observed from the output, javascript didn't wait for the response of the first function and the remaining code block got executed. So callbacks are used in a way to make sure that certain code doesn’t execute until the other code finishes execution.

Answer:

Callback Hell is an anti-pattern with multiple nested callbacks which makes code hard to read and debug when dealing with asynchronous logic. The callback hell looks like below,
 

async1(function(){
    async2(function(){
        async3(function(){
            async4(function(){
                ....
            });
        });
    });
});
 

Answer:

Server-sent events (SSE) is a server push technology enabling a browser to receive automatic updates from a server via HTTP connection without resorting to polling. These are a one way communications channel - events flow from server to client only. This has been used in Facebook/Twitter updates, stock price updates, news feeds etc.

Answer:

The EventSource object is used to receive server-sent event notifications. For example, you can receive messages from server as below,
 

if(typeof(EventSource) !== "undefined") {
  var source = new EventSource("sse_generator.js");
  source.onmessage = function(event) {
    document.getElementById("output").innerHTML += event.data + "
"; }; }
 

Answer:

You can perform browser support for server-sent events before using it as below,
 

if(typeof(EventSource) !== "undefined") {
  // Server-sent events supported. Let's have some code here!
} else {
  // No server-sent events supported
}
 

Answer:

Below are the list of events available for server sent events
 

Event Description
onopen It is used when a connection to the server is opened
onmessage This event is used when a message is received
onerror It happens when an error occurs

Answer:

A promise must follow a specific set of rules,

  1. A promise is an object that supplies a standard-compliant .then() method
  2. A pending promise may transition into either fulfilled or rejected state
  3. A fulfilled or rejected promise is settled and it must not transition into any other state.
  4. Once a promise is settled, the value must not change.

Answer:

You can nest one callback inside in another callback to execute the actions sequentially one by one. This is known as callbacks in callbacks.
 

loadScript('/script1.js', function(script) {
   console.log('first script is loaded');

  loadScript('/script2.js', function(script) {

    console.log('second script is loaded');

    loadScript('/script3.js', function(script) {

        console.log('third script is loaded');
      // after all scripts are loaded
    });

  })

});
 

Answer:

The process of executing a sequence of asynchronous tasks one after another using promises is known as Promise chaining. Let's take an example of promise chaining for calculating the final result,
 

new Promise(function(resolve, reject) {

  setTimeout(() => resolve(1), 1000);

}).then(function(result) {

  console.log(result); // 1
  return result * 2;

}).then(function(result) {

  console.log(result); // 2
  return result * 3;

}).then(function(result) {

  console.log(result); // 6
  return result * 4;

});
 

In the above handlers, the result is passed to the chain of .then() handlers with the below work flow,

  1. The initial promise resolves in 1 second,
  2. After that .then handler is called by logging the result(1) and then return a promise with the value of result * 2.
  3. After that the value passed to the next .then handler by logging the result(2) and return a promise with result * 3.
  4. Finally the value passed to the last .then handler by logging the result(6) and return a promise with result * 4.

Answer:

Promise.all is a promise that takes an array of promises as an input (an iterable), and it gets resolved when all the promises get resolved or any one of them gets rejected. For example, the syntax of promise.all method is below,
 

Promise.all([Promise1, Promise2, Promise3]) .then(result) => {   console.log(result) }) .catch(error => console.log(`Error in promises ${error}`))

Note: Remember that the order of the promises(output the result) is maintained as per input order.

Answer:

Promise.race() method will return the promise instance which is firstly resolved or rejected. Let's take an example of race() method where promise2 is resolved first
 

var promise1 = new Promise(function(resolve, reject) {
    setTimeout(resolve, 500, 'one');
});
var promise2 = new Promise(function(resolve, reject) {
    setTimeout(resolve, 100, 'two');
});

Promise.race([promise1, promise2]).then(function(value) {
  console.log(value); // "two" // Both promises will resolve, but promise2 is faster
});
 

Answer:

Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a “strict” operating context. This way it prevents certain actions from being taken and throws more exceptions. The literal expression "use strict"; instructs the browser to use the javascript code in the Strict mode.

Answer:

Strict mode is useful to write "secure" JavaScript by notifying "bad syntax" into real errors. For example, it eliminates accidentally creating a global variable by throwing an error and also throws an error for assignment to a non-writable property, a getter-only property, a non-existing property, a non-existing variable, or a non-existing object.

Answer:

The strict mode is declared by adding "use strict"; to the beginning of a script or a function. If declared at the beginning of a script, it has global scope.
 

"use strict";
x = 3.14; // This will cause an error because x is not declared

and if you declare inside a function, it has local scope
 
x = 3.14;       // This will not cause an error.
myFunction();

function myFunction() {
  "use strict";
  y = 3.14;   // This will cause an error
}
 

Answer:

The double exclamation or negation(!!) ensures the resulting type is a boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, true. For example, you can test IE version using this expression as below,
 

let isIE8 = false;
isIE8 = !! navigator.userAgent.match(/MSIE 8.0/);
console.log(isIE8); // returns true or false

If you don't use this expression then it returns the original value.
 
console.log(navigator.userAgent.match(/MSIE 8.0/));  // returns either an Array or null

Note: The expression !! is not an operator, but it is just twice of ! operator.

Answer:

The delete keyword is used to delete the property as well as its value.
 

var user= {name: "John", age:20};
delete user.age;

console.log(user); // {name: "John"}
 

Answer:

You can use the JavaScript typeof operator to find the type of a JavaScript variable. It returns the type of a variable or an expression.
 

typeof "John Abraham"     // Returns "string"
typeof (1 + 2)        // Returns "number"
 

Answer:

The undefined property indicates that a variable has not been assigned a value, or not declared at all. The type of undefined value is undefined too.
 

var user;    // Value is undefined, type is undefined
console.log(typeof(user)) //undefined

Any variable can be emptied by setting the value to undefined.
 
user = undefined
 

Answer:

The value null represents the intentional absence of any object value. It is one of JavaScript's primitive values. The type of null value is object. You can empty the variable by setting the value to null.
 

var user = null;
console.log(typeof(user)) //object
 

Answer:

Below are the list of javascript data types available

  1. Number
  2. String
  3. Boolean
  4. Object
  5. Undefined

Answer:

The isNaN() function is used to determine whether a value is an illegal number (Not-a-Number) or not. i.e, This function returns true if the value equates to NaN. Otherwise it returns false.
 

isNaN('Hello') //true
isNaN('100') //false
 

Answer:

Global variables are those that are available throughout the length of the code without any scope. The var keyword is used to declare a local variable but if you omit it then it will become global variable
 

msg = "Hello" // var is missing, it becomes global variable
 

Answer:

The problem with global variables is the conflict of variable names of local and global scope. It is also difficult to debug and test the code that relies on global variables.

Answer:

The NaN property is a global property that represents "Not-a-Number" value. i.e, It indicates that a value is not a legal number. It is very rare to use NaN in a program but it can be used as return value for few cases
 

Math.sqrt(-1)
parseInt("Hello")
 

Answer:

The isFinite() function is used to determine whether a number is a finite, legal number. It returns false if the value is +infinity, -infinity, or NaN (Not-a-Number), otherwise it returns true.
 

isFinite(Infinity);  // false
isFinite(NaN);       // false
isFinite(-Infinity); // false

isFinite(100);         // true
 

Answer:

Event flow is the order in which event is received on the web page. When you click an element that is nested in various other elements, before your click actually reaches its destination, or target element, it must trigger the click event for each of its parent elements first, starting at the top with the global window object. There are two ways of event flow

  1. Top to Bottom(Event Capturing)
  2. Bottom to Top (Event Bubbling)

Answer:

Event bubbling is a type of event propagation where the event first triggers on the innermost target element, and then successively triggers on the ancestors (parents) of the target element in the same nesting hierarchy till it reaches the outermost DOM element.

Answer:

Event capturing is a type of event propagation where the event is first captured by the outermost element, and then successively triggers on the descendants (children) of the target element in the same nesting hierarchy till it reaches the innermost DOM element.

Answer:

You can submit a form using JavaScript use document.form[0].submit(). All the form input's information is submitted using onsubmit event handler
 

function submit() {
    document.form[0].submit();
}
 

Answer:

The window.navigator object contains information about the visitor's browser OS details. Some of the OS properties are available under platform property,
 

console.log(navigator.platform);
 

Answer:

The DOMContentLoaded event is fired when the initial HTML document has been completely loaded and parsed, without waiting for assets(stylesheets, images, and subframes) to finish loading. Whereas The load event is fired when the whole page has loaded, including all dependent resources(stylesheets, images).

Answer:

Native objects are objects that are part of the JavaScript language defined by the ECMAScript specification. For example, String, Math, RegExp, Object, Function etc core objects defined in the ECMAScript spec. Host objects are objects provided by the browser or runtime environment (Node). For example, window, XmlHttpRequest, DOM nodes etc are considered as host objects. User objects are objects defined in the javascript code. For example, User objects created for profile information.

Answer:

You can use below tools or techniques for debugging javascript

  1. Chrome Devtools
  2. debugger statement
  3. Good old console.log statement

Answer:

Below are the list of pros and cons of promises over callbacks,

Pros:

  1. It avoids callback hell which is unreadable
  2. Easy to write sequential asynchronous code with .then()
  3. Easy to write parallel asynchronous code with Promise.all()
  4. Solves some of the common problems of callbacks(call the callback too late, too early, many times and swallow errors/exceptions)

Cons:

  1. It makes little complex code
  2. You need to load a polyfill if ES6 is not supported

Answer:

Attributes are defined on the HTML markup whereas properties are defined on the DOM. For example, the below HTML element has 2 attributes type and value,
 


You can retrieve the attribute value as below,
 
const input = document.querySelector('input');
console.log(input.getAttribute('value')); // Good morning
console.log(input.value); // Good morning

And after you change the value of the text field to "Good evening", it becomes like
 
console.log(input.getAttribute('value')); // Good morning
console.log(input.value); // Good evening
 

Answer:

The same-origin policy is a policy that prevents JavaScript from making requests across domain boundaries. An origin is defined as a combination of URI scheme, hostname, and port number. If you enable this policy then it prevents a malicious script on one page from obtaining access to sensitive data on another web page using Document Object Model(DOM).

Answer:

Void(0) is used to prevent the page from refreshing. This will be helpful to eliminate the unwanted side-effect, because it will return the undefined primitive value. It is commonly used for HTML documents that use href="JavaScript:Void(0);" within an  element. i.e, when you click a link, the browser loads a new page or refreshes the same page. But this behavior will be prevented using this expression. For example, the below link notify the message without reloading the page
 

Click Me!
 

Answer:

JavaScript is an interpreted language, not a compiled language. An interpreter in the browser reads over the JavaScript code, interprets each line, and runs it. Nowadays modern browsers use a technology known as Just-In-Time (JIT) compilation, which compiles JavaScript to executable bytecode just as it is about to run.

Answer:

Yes, JavaScript is a case sensitive language. The language keywords, variables, function & object names, and any other identifiers must always be typed with a consistent capitalization of letters.

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No, they are entirely two different programming languages and have nothing to do with each other. But both of them are Object Oriented Programming languages and like many other languages, they follow similar syntax for basic features(if, else, for, switch, break, continue etc).

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Events are "things" that happen to HTML elements. When JavaScript is used in HTML pages, JavaScript can react on these events. Some of the examples of HTML events are,

  1. Web page has finished loading
  2. Input field was changed
  3. Button was clicked

Let's describe the behavior of click event for button element,

 



 
   
 
 
   
 

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JavaScript was created by Brendan Eich in 1995 during his time at Netscape Communications. Initially it was developed under the name Mocha, but later the language was officially called LiveScript when it first shipped in beta releases of Netscape.

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The preventDefault() method cancels the event if it is cancelable, meaning that the default action or behaviour that belongs to the event will not occur. For example, prevent form submission when clicking on submit button and prevent opening the page URL when clicking on hyperlink are some common use cases.
 

document.getElementById("link").addEventListener("click", function(event){
 event.preventDefault();
});

Note: Remember that not all events are cancelable.

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The stopPropagation method is used to stop the event from bubbling up the event chain. For example, the below nested divs with stopPropagation method prevents default event propagation when clicking on nested div(Div1)
 

Click DIV1 Element

DIV 2
DIV 1
function firstFunc(event) { alert("DIV 1"); event.stopPropagation(); } function secondFunc() { alert("DIV 2"); }
 

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The return false statement in event handlers performs the below steps,

  1. First it stops the browser's default action or behaviour.
  2. It prevents the event from propagating the DOM
  3. Stops callback execution and returns immediately when called.

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The Browser Object Model (BOM) allows JavaScript to "talk to" the browser. It consists of the objects navigator, history, screen, location and document which are children of the window. The Browser Object Model is not standardized and can change based on different browsers.

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The setTimeout() method is used to call a function or evaluate an expression after a specified number of milliseconds. For example, let's log a message after 2 seconds using setTimeout method,
 

setTimeout(function(){ console.log("Good morning"); }, 2000);
 

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The setInterval() method is used to call a function or evaluate an expression at specified intervals (in milliseconds). For example, let's log a message after 2 seconds using setInterval method,
 

setInterval(function(){ console.log("Good morning"); }, 2000);
 

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JavaScript is a single-threaded language. Because the language specification does not allow the programmer to write code so that the interpreter can run parts of it in parallel in multiple threads or processes. Whereas languages like java, go, C++ can make multi-threaded and multi-process programs.

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Event delegation is a technique for listening to events where you delegate a parent element as the listener for all of the events that happen inside it.

For example, if you wanted to detect field changes in inside a specific form, you can use event delegation technique,

 

var form = document.querySelector('#registration-form');

// Listen for changes to fields inside the form
form.addEventListener('input', function (event) {

// Log the field that was changed
console.log(event.target);

}, false);

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ECMAScript is the scripting language that forms the basis of JavaScript. ECMAScript standardized by the ECMA International standards organization in the ECMA-262 and ECMA-402 specifications. The first edition of ECMAScript was released in 1997.

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JSON (JavaScript Object Notation) is a lightweight format that is used for data interchanging. It is based on a subset of JavaScript language in the way objects are built in JavaScript.

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Below are the list of syntax rules of JSON

  1. The data is in name/value pairs
  2. The data is separated by commas
  3. Curly braces hold objects
  4. Square brackets hold arrays

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When sending data to a web server, the data has to be in a string format. You can achieve this by converting JSON object into a string using stringify() method.
 

var userJSON = {'name': 'John', age: 31}
var userString = JSON.stringify(user);
console.log(userString); //"{"name":"John","age":31}"
 

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When receiving the data from a web server, the data is always in a string format. But you can convert this string value to a javascript object using parse() method.
 

var userString = '{"name":"John","age":31}';
var userJSON = JSON.parse(userString);
console.log(userJSON);// {name: "John", age: 31}
 

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When exchanging data between a browser and a server, the data can only be text. Since JSON is text only, it can easily be sent to and from a server, and used as a data format by any programming language.

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Progressive web applications (PWAs) are a type of mobile app delivered through the web, built using common web technologies including HTML, CSS and JavaScript. These PWAs are deployed to servers, accessible through URLs, and indexed by search engines.

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The clearTimeout() function is used in javascript to clear the timeout which has been set by setTimeout()function before that. i.e, The return value of setTimeout() function is stored in a variable and it’s passed into the clearTimeout() function to clear the timer.

For example, the below setTimeout method is used to display the message after 3 seconds. This timeout can be cleared by the clearTimeout() method.

 

var msg;
function greeting() {
   alert('Good morning');
}
function start() {
  msg =setTimeout(greeting, 3000);

}

function stop() {
    clearTimeout(msg);
}

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The clearInterval() function is used in javascript to clear the interval which has been set by setInterval() function. i.e, The return value returned by setInterval() function is stored in a variable and it’s passed into the clearInterval() function to clear the interval.

For example, the below setInterval method is used to display the message for every 3 seconds. This interval can be cleared by the clearInterval() method.

 

var msg;
function greeting() {
   alert('Good morning');
}
function start() {
  msg = setInterval(greeting, 3000);

}

function stop() {
    clearInterval(msg);
}

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In vanilla javascript, you can redirect to a new page using the location property of window object. The syntax would be as follows,
 

function redirect() {
   window.location.href = 'newPage.html';
}
 

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There are 3 possible ways to check whether a string contains a substring or not,

1. Using includes: ES6 provided String.prototype.includes method to test a string contains a substring
 

var mainString = "hello", subString = "hell";
mainString.includes(subString)

2. Using indexOf: In an ES5 or older environment, you can use String.prototype.indexOf which returns the index of a substring. If the index value is not equal to -1 then it means the substring exists in the main string.
 
var mainString = "hello", subString = "hell";
mainString.indexOf(subString) !== -1

3. Using RegEx: The advanced solution is using Regular expression's test method(RegExp.test), which allows for testing for against regular expressions
 
var mainString = "hello", regex = "/hell/";
regex.test(mainString)
 

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You can validate an email in javascript using regular expressions. It is recommended to do validations on the server side instead of the client side. Because the javascript can be disabled on the client side.
 

function validateEmail(email) {
    var re = /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
    return re.test(String(email).toLowerCase());
}

The above regular expression accepts unicode characters.

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You can use window.location.href expression to get the current url path and you can use the same expression for updating the URL too. You can also use document.URL for read-only purposes but this solution has issues in FF.
 

console.log('location.href', window.location.href); // Returns full URL
 

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The below Location object properties can be used to access URL components of the page,

  1. href - The entire URL
  2. protocol - The protocol of the URL
  3. host - The hostname and port of the URL
  4. hostname - The hostname of the URL
  5. port - The port number in the URL
  6. pathname - The path name of the URL
  7. search - The query portion of the URL
  8. hash - The anchor portion of the URL

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You can use URLSearchParams to get query string values in javascript. Let's see an example to get the client code value from URL query string,
 

const urlParams = new URLSearchParams(window.location.search);
const clientCode = urlParams.get('clientCode');
 

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You can check whether a key exists in an object or not using three approaches,

1. Using in operator: You can use the in operator whether a key exists in an object or not
 

"key" in obj
and If you want to check if a key doesn't exist, remember to use parenthesis,
!("key" in obj)

2. Using hasOwnProperty method: You can use hasOwnProperty to particularly test for properties of the object instance (and not inherited properties)
obj.hasOwnProperty("key") // true

I. Using undefined comparison: If you access a non-existing property from an object, the result is undefined. Let’s compare the properties against undefined to determine the existence of the property.
 
const user = {
  name: 'John'
};

console.log(user.name !== undefined);     // true
console.log(user.nickName !== undefined); // false
 

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You can use the for-in loop to loop through javascript object. You can also make sure that the key you get is an actual property of an object, and doesn't come from the prototype using hasOwnProperty method.
 

var object = {
    "k1": "value1",
    "k2": "value2",
    "k3": "value3"
};

for (var key in object) {
    if (object.hasOwnProperty(key)) {
        console.log(key + " -> " + object[key]); // k1 -> value1 ...
    }
}
 

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There are different solutions based on ECMAScript versions
1. Using Object entries(ECMA 7+): You can use object entries length along with constructor type.

Object.entries(obj).length === 0 && obj.constructor === Object // Since date object length is 0, you need to check constructor check as well
2. Using Object keys(ECMA 5+): You can use object keys length along with constructor type.
Object.keys(obj).length === 0 && obj.constructor === Object // Since date object length is 0, you need to check constructor check as well
3. Using for-in with hasOwnProperty(Pre-ECMA 5): You can use a for-in loop along with hasOwnProperty.
function isEmpty(obj) {
  for(var prop in obj) {
    if(obj.hasOwnProperty(prop)) {
      return false;
    }
  }

  return JSON.stringify(obj) === JSON.stringify({});
}
 

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The arguments object is an Array-like object accessible inside functions that contains the values of the arguments passed to that function. For example, let's see how to use arguments object inside sum function,
 

function sum() {
    var total = 0;
    for (var i = 0, len = arguments.length; i < len; ++i) {
        total += arguments[i];
    }
    return total;
}

sum(1, 2, 3) // returns 6
Note: You can't apply array methods on arguments object. But you can convert into a regular array as below.
 
var argsArray = Array.prototype.slice.call(arguments);
 

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You can create a function which uses a chain of string methods such as charAt, toUpperCase and slice methods to generate a string with the first letter in uppercase.
 

function capitalizeFirstLetter(string) {
    return string.charAt(0).toUpperCase() + string.slice(1);
}
 

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The for-loop is a commonly used iteration syntax in javascript. It has both pros and cons
Pros :

  1. Works on every environment
  2. You can use break and continue flow control statements

Cons :

  1. Too verbose
  2. Imperative
  3. You might face one-by-off errors

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You can use new Date() to generate a new Date object containing the current date and time. For example, let's display the current date in mm/dd/yyyy
 

var today = new Date();
var dd = String(today.getDate()).padStart(2, '0');
var mm = String(today.getMonth() + 1).padStart(2, '0'); //January is 0!
var yyyy = today.getFullYear();

today = mm + '/' + dd + '/' + yyyy;
document.write(today);
 

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You need to use date.getTime() method to compare date values instead of comparison operators (==, !=, ===, and !== operators)
 

var d1 = new Date();
var d2 = new Date(d1);
console.log(d1.getTime() === d2.getTime()); //True
console.log(d1 === d2); // False
 

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You can use ECMAScript 6's String.prototype.startsWith() method to check if a string starts with another string or not. But it is not yet supported in all browsers. Let's see an example to see this usage,
 

"Good morning".startsWith("Good"); // true
"Good morning".startsWith("morning"); // false
 

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JavaScript provided a trim method on string types to trim any whitespaces present at the beginning or ending of the string.
 

"  Hello World   ".trim(); //Hello World
If your browser(  
if (!String.prototype.trim) {
    (function() {
        // Make sure we trim BOM and NBSP
        var rtrim = /^[\s\uFEFF\xA0]+|[\s\uFEFF\xA0]+$/g;
        String.prototype.trim = function() {
            return this.replace(rtrim, '');
        };
    })();
}
 

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There are two possible solutions to add new properties to an object. Let's take a simple object to explain these solutions.
 

var object = {
    key1: value1,
    key2: value2
};

1. Using dot notation: This solution is useful when you know the name of the property
 
object.key3 = "value3";

2. Using square bracket notation: This solution is useful when the name of the property is dynamically determined.
 
obj["key3"] = "value3";
 

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No,that's not a special operator. But it is a combination of 2 standard operators one after the other,

  1. A logical not (!)
  2. A prefix decrement (--)

At first, the value decremented by one and then tested to see if it is equal to zero or not for determining the truthy/falsy value.

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You can use the logical or operator || in an assignment expression to provide a default value. The syntax looks like as below,
 

var a = b || c;

As per the above expression, variable 'a 'will get the value of 'c' only if 'b' is falsy (if is null, false, undefined, 0, empty string, or NaN), otherwise 'a' will get the value of 'b'.

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You can define multiline string literals using the '' character followed by line terminator.
 

var str = "This is a \
very lengthy \
sentence!";
But if you have a space after the '' character, the code will look exactly the same, but it will raise a SyntaxError.

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An application shell (or app shell) architecture is one way to build a Progressive Web App that reliably and instantly loads on your users' screens, similar to what you see in native applications. It is useful for getting some initial HTML to the screen fast without a network.

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Yes, We can define properties for functions because functions are also objects.
 

fn = function(x) {
   //Function code goes here
}

fn.name = "John";

fn.profile = function(y) {
  //Profile code goes here
}
 

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You can use function.length syntax to find the number of parameters expected by a function. Let's take an example of sum function to calculate the sum of numbers,
 

function sum(num1, num2, num3, num4){
    return num1 + num2 + num3 + num4;
}
sum.length // 4 is the number of parameters expected.
 

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A polyfill is a piece of JS code used to provide modern functionality on older browsers that do not natively support it. For example, Silverlight plugin polyfill can be used to mimic the functionality of an HTML Canvas element on Microsoft Internet Explorer 7.

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The break statement is used to "jump out" of a loop. i.e, It breaks the loop and continues executing the code after the loop.
 

for (i = 0; i < 10; i++) {
  if (i === 5) { break; }
  text += "Number: " + i + "
"; }

The continue statement is used to "jump over" one iteration in the loop. i.e, It breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.
 
for (i = 0; i < 10; i++) {
    if (i === 5) { continue; }
    text += "Number: " + i + "
"; }
 

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The label statement allows us to name loops and blocks in JavaScript. We can then use these labels to refer back to the code later. For example, the below code with labels avoids printing the numbers when they are same,
 

var i, j;

loop1:
for (i = 0; i < 3; i++) {
   loop2:
   for (j = 0; j < 3; j++) {
      if (i === j) {
         continue loop1;
      }
      console.log('i = ' + i + ', j = ' + j);
   }
}

// Output is:
//   "i = 1, j = 0"
//   "i = 2, j = 0"
//   "i = 2, j = 1"
 

Answer:

It is recommended to keep all declarations at the top of each script or function. The benefits of doing this are,

  1. Gives cleaner code
  2. It provides a single place to look for local variables
  3. Easy to avoid unwanted global variables
  4. It reduces the possibility of unwanted re-declarations

Answer:

It is recommended to initialize variables because of the below benefits,

  1. It gives cleaner code
  2. It provides a single place to initialize variables
  3. Avoid undefined values in the code

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It is recommended to avoid creating new objects using new Object(). Instead you can initialize values based on it's type to create the objects.

  1. Assign {} instead of new Object()
  2. Assign "" instead of new String()
  3. Assign 0 instead of new Number()
  4. Assign false instead of new Boolean()
  5. Assign [] instead of new Array()
  6. Assign /()/ instead of new RegExp()
  7. Assign function (){} instead of new Function()

You can define them as an example

 

var v1 = {};
var v2 = "";
var v3 = 0;
var v4 = false;
var v5 = [];
var v6 = /()/;
var v7 = function(){};

Answer:

JSON arrays are written inside square brackets and arrays contain javascript objects. For example, the JSON array of users would be as below,
 

"users":[
  {"firstName":"John", "lastName":"Abrahm"},
  {"firstName":"Anna", "lastName":"Smith"},
  {"firstName":"Shane", "lastName":"Warn"}
]
 

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You can use Math.random() with Math.floor() to return random integers. For example, if you want generate random integers between 1 to 10, the multiplication factor should be 10,
 

Math.floor(Math.random() * 10) + 1;     // returns a random integer from 1 to 10
Math.floor(Math.random() * 100) + 1;     // returns a random integer from 1 to 100
 

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Yes, you can create a proper random function to return a random number between min and max (both included)
 

function randomInteger(min, max) {
  return Math.floor(Math.random() * (max - min + 1) ) + min;
}
randomInteger(1, 100); // returns a random integer from 1 to 100
randomInteger(1, 1000); // returns a random integer from 1 to 1000
 

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Tree shaking is a form of dead code elimination. It means that unused modules will not be included in the bundle during the build process and for that it relies on the static structure of ES2015 module syntax,( i.e. import and export). Initially this has been popularized by the ES2015 module bundler rollup.

Answer:

Tree Shaking can significantly reduce the code size in any application. i.e, The less code we send over the wire the more performant the application will be. For example, if we just want to create a “Hello World” Application using SPA frameworks then it will take around a few MBs, but by tree shaking it can bring down the size to just a few hundred KBs. Tree shaking is implemented in Rollup and Webpack bundlers.

Answer:

No, it allows arbitrary code to be run which causes a security problem. As we know that the eval() function is used to run text as code. In most of the cases, it should not be necessary to use it.

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A regular expression is a sequence of characters that forms a search pattern. You can use this search pattern for searching data in a text. These can be used to perform all types of text search and text replace operations. Let's see the syntax format now,
 

/pattern/modifiers;
For example, the regular expression or search pattern with case-insensitive username would be,
 
/John/i
 

Answer:

Regular Expressions has two string methods: search() and replace(). The search() method uses an expression to search for a match, and returns the position of the match.
 

var msg = "Hello John";
var n = msg.search(/John/i); // 6

The replace() method is used to return a modified string where the pattern is replaced.
 
var msg = "Hello John";
var n = msg.replace(/John/i, "Buttler"); // Hello Buttler
 

Answer:

Modifiers can be used to perform case-insensitive and global searches. Let's list down some of the modifiers,
 

Modifier Description
i Perform case-insensitive matching
g Perform a global match rather than stops at first match
m Perform multiline matching

Let's take an example of global modifier,
 
var text = "Learn JS one by one";
var pattern = /one/g;
var result = text.match(pattern); // one,one
 

Answer:

Regular Expressions provide a group of patterns in order to match characters. Basically they are categorized into 3 types,

  1. Brackets: These are used to find a range of characters. For example, below are some use cases,
    1. [abc]: Used to find any of the characters between the brackets(a,b,c)
    2. [0-9]: Used to find any of the digits between the brackets
    3. (a|b): Used to find any of the alternatives separated with |
  2. Metacharacters: These are characters with a special meaning For example, below are some use cases,
    1. \d: Used to find a digit
    2. \s: Used to find a whitespace character
    3. \b: Used to find a match at the beginning or ending of a word
  3. Quantifiers: These are useful to define quantities For example, below are some use cases,
    1. n+: Used to find matches for any string that contains at least one n
    2. n*: Used to find matches for any string that contains zero or more occurrences of n
    3. n?: Used to find matches for any string that contains zero or one occurrences of n

Answer:

RegExp object is a regular expression object with predefined properties and methods. Let's see the simple usage of RegExp object,
 

var regexp = new RegExp('\\w+');
console.log(regexp);
// expected output: /\w+/
 

Answer:

You can use the test() method of regular expression in order to search a string for a pattern, and return true or false depending on the result.
 

var pattern = /you/;
console.log(pattern.test("How are you?")); //true
 

Answer:

The purpose of exec method is similar to test method but it executes a search for a match in a specified string and returns a result array, or null instead of returning true/false.
 

var pattern = /you/;
console.log(pattern.exec("How are you?")); //["you", index: 8, input: "How are you?", groups: undefined]
 

Answer:

You can change inline style or classname of a HTML element using javascript

1. Using style property: You can modify inline style using style property

document.getElementById("title").style.fontSize = "30px";
2. Using ClassName property: It is easy to modify element class using className property
document.getElementById("title").style.className = "custom-title";
 

Answer:

The output is going to be 33. Since 1 and 2 are numeric values, the result of the first two digits is going to be a numeric value 3. The next digit is a string type value because of that the addition of numeric value 3 and string type value 3 is just going to be a concatenation value 33.

Answer:

The debugger statement invokes any available debugging functionality, such as setting a breakpoint. If no debugging functionality is available, this statement has no effect. For example, in the below function a debugger statement has been inserted. So execution is paused at the debugger statement just like a breakpoint in the script source.
 

function getProfile() {
// code goes here
debugger;
// code goes here
}
 

Answer:

You can set breakpoints in the javascript code once the debugger statement is executed and the debugger window pops up. At each breakpoint, javascript will stop executing, and let you examine the JavaScript values. After examining values, you can resume the execution of code using the play button.

Answer:

No, you cannot use the reserved words as variables, labels, object or function names. Let's see one simple example,
 

var else = "hello"; // Uncaught SyntaxError: Unexpected token else
 

Answer:

You can use regex which returns a true or false value depending on whether or not the user is browsing with a mobile.
 

window.mobilecheck = function() {
  var mobileCheck = false;
  (function(a){if(/(android|bb\d+|meego).+mobile|avantgo|bada\/|blackberry|blazer|compal|elaine|fennec|hiptop|iemobile|ip(hone|od)|iris|kindle|lge |maemo|midp|mmp|mobile.+firefox|netfront|opera m(ob|in)i|palm( os)?|phone|p(ixi|re)\/|plucker|pocket|psp|series(4|6)0|symbian|treo|up\.(browser|link)|vodafone|wap|windows ce|xda|xiino/i.test(a)||/1207|6310|6590|3gso|4thp|50[1-6]i|770s|802s|a wa|abac|ac(er|oo|s\-)|ai(ko|rn)|al(av|ca|co)|amoi|an(ex|ny|yw)|aptu|ar(ch|go)|as(te|us)|attw|au(di|\-m|r |s )|avan|be(ck|ll|nq)|bi(lb|rd)|bl(ac|az)|br(e|v)w|bumb|bw\-(n|u)|c55\/|capi|ccwa|cdm\-|cell|chtm|cldc|cmd\-|co(mp|nd)|craw|da(it|ll|ng)|dbte|dc\-s|devi|dica|dmob|do(c|p)o|ds(12|\-d)|el(49|ai)|em(l2|ul)|er(ic|k0)|esl8|ez([4-7]0|os|wa|ze)|fetc|fly(\-|_)|g1 u|g560|gene|gf\-5|g\-mo|go(\.w|od)|gr(ad|un)|haie|hcit|hd\-(m|p|t)|hei\-|hi(pt|ta)|hp( i|ip)|hs\-c|ht(c(\-| |_|a|g|p|s|t)|tp)|hu(aw|tc)|i\-(20|go|ma)|i230|iac( |\-|\/)|ibro|idea|ig01|ikom|im1k|inno|ipaq|iris|ja(t|v)a|jbro|jemu|jigs|kddi|keji|kgt( |\/)|klon|kpt |kwc\-|kyo(c|k)|le(no|xi)|lg( g|\/(k|l|u)|50|54|\-[a-w])|libw|lynx|m1\-w|m3ga|m50\/|ma(te|ui|xo)|mc(01|21|ca)|m\-cr|me(rc|ri)|mi(o8|oa|ts)|mmef|mo(01|02|bi|de|do|t(\-| |o|v)|zz)|mt(50|p1|v )|mwbp|mywa|n10[0-2]|n20[2-3]|n30(0|2)|n50(0|2|5)|n7(0(0|1)|10)|ne((c|m)\-|on|tf|wf|wg|wt)|nok(6|i)|nzph|o2im|op(ti|wv)|oran|owg1|p800|pan(a|d|t)|pdxg|pg(13|\-([1-8]|c))|phil|pire|pl(ay|uc)|pn\-2|po(ck|rt|se)|prox|psio|pt\-g|qa\-a|qc(07|12|21|32|60|\-[2-7]|i\-)|qtek|r380|r600|raks|rim9|ro(ve|zo)|s55\/|sa(ge|ma|mm|ms|ny|va)|sc(01|h\-|oo|p\-)|sdk\/|se(c(\-|0|1)|47|mc|nd|ri)|sgh\-|shar|sie(\-|m)|sk\-0|sl(45|id)|sm(al|ar|b3|it|t5)|so(ft|ny)|sp(01|h\-|v\-|v )|sy(01|mb)|t2(18|50)|t6(00|10|18)|ta(gt|lk)|tcl\-|tdg\-|tel(i|m)|tim\-|t\-mo|to(pl|sh)|ts(70|m\-|m3|m5)|tx\-9|up(\.b|g1|si)|utst|v400|v750|veri|vi(rg|te)|vk(40|5[0-3]|\-v)|vm40|voda|vulc|vx(52|53|60|61|70|80|81|83|85|98)|w3c(\-| )|webc|whit|wi(g |nc|nw)|wmlb|wonu|x700|yas\-|your|zeto|zte\-/i.test(a.substr(0,4))) mobileCheck = true;})(navigator.userAgent||navigator.vendor||window.opera);
  return mobileCheck;
};
 

Answer:

You can detect mobile browsers by simply running through a list of devices and checking if the useragent matches anything. This is an alternative solution for RegExp usage,
 

function detectmob() {
 if( navigator.userAgent.match(/Android/i)
 || navigator.userAgent.match(/webOS/i)
 || navigator.userAgent.match(/iPhone/i)
 || navigator.userAgent.match(/iPad/i)
 || navigator.userAgent.match(/iPod/i)
 || navigator.userAgent.match(/BlackBerry/i)
 || navigator.userAgent.match(/Windows Phone/i)
 ){
    return true;
  }
 else {
    return false;
  }
}
 

Answer:

You can programmatically get the image and check the dimensions(width and height) using Javascript.
 

var img = new Image();
img.onload = function() {
  console.log(this.width + 'x' + this.height);
}
img.src = 'http://www.google.com/intl/en_ALL/images/logo.gif';
 

Answer:

Browsers provide an XMLHttpRequest object which can be used to make synchronous HTTP requests from JavaScript
 

function httpGet(theUrl)
{
    var xmlHttpReq = new XMLHttpRequest();
    xmlHttpReq.open( "GET", theUrl, false ); // false for synchronous request
    xmlHttpReq.send( null );
    return xmlHttpReq.responseText;
}
 

Answer:

Browsers provide an XMLHttpRequest object which can be used to make asynchronous HTTP requests from JavaScript by passing the 3rd parameter as true.
 

function httpGetAsync(theUrl, callback)
{
    var xmlHttpReq = new XMLHttpRequest();
    xmlHttpReq.onreadystatechange = function() {
        if (xmlHttpReq.readyState == 4 && xmlHttpReq.status == 200)
            callback(xmlHttpReq.responseText);
    }
    xmlHttp.open("GET", theUrl, true); // true for asynchronous
    xmlHttp.send(null);
}
 

Answer:

You can use the toLocaleString() method to convert dates in one timezone to another. For example, let's convert current date to British English timezone as below,
 

console.log(event.toLocaleString('en-GB', { timeZone: 'UTC' })); //29/06/2019, 09:56:00
 

Answer:

You can use innerWidth, innerHeight, clientWidth, clientHeight properties of windows, document element and document body objects to find the size of a window. Let's use them combination of these properties to calculate the size of a window or document,
 

var width = window.innerWidth
|| document.documentElement.clientWidth
|| document.body.clientWidth;

var height = window.innerHeight
|| document.documentElement.clientHeight
|| document.body.clientHeight;
 

Answer:

The conditional (ternary) operator is the only JavaScript operator that takes three operands which acts as a shortcut for if statements.
 

var isAuthenticated = false;
console.log(isAuthenticated ? 'Hello, welcome' : 'Sorry, you are not authenticated'); //Sorry, you are not authenticated
 

Answer:

Yes, you can apply chaining on conditional operators similar to if … else if?… else if … else chain. The syntax is going to be as below,
 

function traceValue(someParam) {
    return condition1 ? value1
         : condition2 ? value2
         : condition3 ? value3
         : value4;
}

// The above conditional operator is equivalent to:

function traceValue(someParam) {
    if (condition1) { return value1; }
    else if (condition2) { return value2; }
    else if (condition3) { return value3; }
    else { return value4; }
}
 

Answer:

You can execute javascript after page load in many different ways,

1. window.onload:

window.onload = function ...
2. document.onload:
document.onload = function ...
3. body onload:
 

Answer:

he __proto__ object is the actual object that is used in the lookup chain to resolve methods, etc. Whereas prototype is the object that is used to build __proto__ when you create an object with new
 

( new Employee ).__proto__ === Employee.prototype;
( new Employee ).prototype === undefined;
 

Answer:

It is recommended to use semicolons after every statement in JavaScript. For example, in the below case it throws an error ".. is not a function" at runtime due to missing semicolon.
 

// define a function
var fn = function () {
    //...
} // semicolon missing at this line

// then execute some code inside a closure
(function () {
    //...
})();
and it will be interpreted as
var fn = function () {
    //...
}(function () {
    //...
})();
In this case, we are passing the second function as an argument to the first function and then trying to call the result of the first function call as a function. Hence, the second function will fail with a "... is not a function" error at runtime.

Answer:

The freeze() method is used to freeze an object. Freezing an object does not allow adding new properties to an object,prevents from removing and prevents changing the enumerability, configurability, or writability of existing properties. i.e, It returns the passed object and does not create a frozen copy.
 

const obj = {
  prop: 100
};

Object.freeze(obj);
obj.prop = 200; // Throws an error in strict mode

console.log(obj.prop); //100
Note: It causes a TypeError if the argument passed is not an object.

Answer:

Below are the main benefits of using freeze method,

  1. It is used for freezing objects and arrays.
  2. It is used to make an object immutable.

Answer:

In the Object-oriented paradigm, an existing API contains certain elements that are not intended to be extended, modified, or re-used outside of their current context. Hence it works as the final keyword which is used in various languages.

Answer:

You can use navigator object to detect a browser language preference as below,
 

var language = navigator.languages && navigator.languages[0] || // Chrome / Firefox
               navigator.language ||   // All browsers
               navigator.userLanguage; // IE <= 10

console.log(language);
 

Answer:

Title case means that the first letter of each word is capitalized. You can convert a string to title case using the below function,
 

unction toTitleCase(str) {
        return str.replace(
            /\w\S*/g,
            function(txt) {
                return txt.charAt(0).toUpperCase() + txt.substr(1).toLowerCase();
            }
        );
    }
    toTitleCase("good morning john"); // Good Morning John
 

Answer:

You can use the 


 

Answer:

An operator is capable of manipulating(mathematical and logical computations) a certain value or operand. There are various operators supported by JavaScript as below,

  1. Arithmetic Operators: Includes + (Addition),– (Subtraction), * (Multiplication), / (Division), % (Modulus), + + (Increment) and – – (Decrement)
  2. Comparison Operators: Includes = =(Equal),!= (Not Equal), ===(Equal with type), > (Greater than),> = (Greater than or Equal to),< (Less than),<= (Less than or Equal to)
  3. Logical Operators: Includes &&(Logical AND),||(Logical OR),!(Logical NOT)
  4. Assignment Operators: Includes = (Assignment Operator), += (Add and Assignment Operator), – = (Subtract and Assignment Operator), *= (Multiply and Assignment), /= (Divide and Assignment), %= (Modules and Assignment)
  5. Ternary Operators: It includes conditional(: ?) Operator
  6. typeof Operator: It uses to find type of variable. The syntax looks like typeof variable

Answer:

Rest parameter is an improved way to handle function parameters which allows us to represent an indefinite number of arguments as an array. The syntax would be as below,
 

function f(a, b, ...theArgs) {
  // ...
}
For example, let's take a sum example to calculate on dynamic number of parameters,
 
function total(…args){
let sum = 0;
for(let i of args){
sum+=i;
}
return sum;
}
console.log(fun(1,2)); //3
console.log(fun(1,2,3)); //6
console.log(fun(1,2,3,4)); //13
console.log(fun(1,2,3,4,5)); //15
Note: Rest parameter is added in ES2015 or ES6

Answer:

The rest parameter should be the last argument, as its job is to collect all the remaining arguments into an array. For example, if you define a function like below it doesn’t make any sense and will throw an error.
 

function someFunc(a,…b,c){
//You code goes here
return;
}
 

Answer:

Below are the list of bitwise logical operators used in JavaScript

  1. Bitwise AND ( & )
  2. Bitwise OR ( | )
  3. Bitwise XOR ( ^ )
  4. Bitwise NOT ( ~ )
  5. Left Shift ( << )
  6. Sign Propagating Right Shift ( >> )
  7. Zero fill Right Shift ( >>> )

Answer:

Spread operator allows iterables( arrays / objects / strings ) to be expanded into single arguments/elements. Let's take an example to see this behavior,
 

function calculateSum(x, y, z) {
  return x + y + z;
}

const numbers = [1, 2, 3];

console.log(calculateSum(...numbers)); // 6
 

Answer:

Object.isFrozen() method is used to determine if an object is frozen or not.An object is frozen if all of the below conditions hold true,

  1. If it is not extensible.
  2. If all of its properties are non-configurable.
  3. If all its data properties are non-writable. The usage is going to be as follows,
const object = {
   property: 'Welcome JS world'
};
Object.freeze(object);
console.log(Object.isFrozen(object));
 

Answer:

The Object.is() method determines whether two values are the same value. For example, the usage with different types of values would be,
 

Object.is('hello', 'hello');     // true
Object.is(window, window);   // true
Object.is([], []) // false
 

Two values are the same if one of the following holds:

  1. both undefined
  2. both null
  3. both true or both false
  4. both strings of the same length with the same characters in the same order
  5. both the same object (means both object have same reference)
  6. both numbers and both +0 both -0 both NaN both non-zero and both not NaN and both have the same value.

Answer:

Some of the applications of Object's is method are follows,

  1. It is used for comparison of two strings.
  2. It is used for comparison of two numbers.
  3. It is used for comparing the polarity of two numbers.
  4. It is used for comparison of two objects.

Answer:

You can use the Object.assign() method which is used to copy the values and properties from one or more source objects to a target object. It returns the target object which has properties and values copied from the target object. The syntax would be as below,
 

Object.assign(target, ...sources)
Let's take example with one source and one target object,
 
const target = { a: 1, b: 2 };
const source = { b: 3, c: 4 };

const returnedTarget = Object.assign(target, source);

console.log(target); // { a: 1, b: 3, c: 4 }

console.log(returnedTarget); // { a: 1, b: 3, c: 4 }
As observed in the above code, there is a common property(b) from source to target so it's value has been overwritten.

Answer:

Below are the some of main applications of Object.assign() method,

  1. It is used for cloning an object.
  2. It is used to merge objects with the same properties.

Answer:

The Proxy object is used to define custom behavior for fundamental operations such as property lookup, assignment, enumeration, function invocation, etc. The syntax would be as follows,
 

var p = new Proxy(target, handler);
Let's take an example of proxy object,
 
var handler = {
    get: function(obj, prop) {
        return prop in obj ?
            obj[prop] :
            100;
    }
};

var p = new Proxy({}, handler);
p.a = 10;
p.b = null;

console.log(p.a, p.b); // 10, null
console.log('c' in p, p.c); // false, 100
In the above code, it uses get handler which define the behavior of the proxy when an operation is performed on it

Answer:

The Object.seal() method is used to seal an object, by preventing new properties from being added to it and marking all existing properties as non-configurable. But values of present properties can still be changed as long as they are writable. Let's see the below example to understand more about seal() method
 

const object = {
    property: 'Welcome JS world'
 };
 Object.seal(object);
 object.property = 'Welcome to object world';
 console.log(Object.isSealed(object)); // true
 delete object.property; // You cannot delete when sealed
 console.log(object.property); //Welcome to object world
 

Answer:

Below are the main applications of Object.seal() method,

  1. It is used for sealing objects and arrays.
  2. It is used to make an object immutable.

Answer:

If an object is frozen using the Object.freeze() method then its properties become immutable and no changes can be made in them whereas if an object is sealed using the Object.seal() method then the changes can be made in the existing properties of the object.

Answer:

The Object.isSealed() method is used to determine if an object is sealed or not. An object is sealed if all of the below conditions hold true

  1. If it is not extensible.
  2. If all of its properties are non-configurable.
  3. If it is not removable (but not necessarily non-writable). Let's see it in the action
const object = {
property: 'Hello, Good morning'
};

Object.seal(object); // Using seal() method to seal the object

console.log(Object.isSealed(object));      // checking whether the object is sealed or not
 

Answer:

The Object.entries() method is used to return an array of a given object's own enumerable string-keyed property [key, value] pairs, in the same order as that provided by a for...in loop. Let's see the functionality of object.entries() method in an example,
 

const object = {
  a: 'Good morning',
  b: 100
};

for (let [key, value] of Object.entries(object)) {
  console.log(`${key}: ${value}`); // a: 'Good morning'
                                   // b: 100
}
Note: The order is not guaranteed as object defined.

Answer:

The Object.values() method's behavior is similar to Object.entries() method but it returns an array of values instead [key,value] pairs.
 

const object = {
   a: 'Good morning',
   b: 100
 };

 for (let value of Object.values(object)) {
   console.log(`${value}`); // 'Good morning'
                                100
 }
 

Answer:

You can use the Object.keys() method which is used to return an array of a given object's own property names, in the same order as we get with a normal loop. For example, you can get the keys of a user object,
 

const user = {
  name: 'John',
  gender: 'male',
  age: 40
};

console.log(Object.keys(user)); //['name', 'gender', 'age']
 

Answer:

The Object.create() method is used to create a new object with the specified prototype object and properties. i.e, It uses an existing object as the prototype of the newly created object. It returns a new object with the specified prototype object and properties.
 

const user = {
   name: 'John',
   printInfo: function () {
     console.log(`My name is ${this.name}.`);
   }
 };

 const admin = Object.create(user);

 admin.name = "Nick"; // Remember that "name" is a property set on "admin" but not on "user" object

 admin.printInfo(); // My name is Nick
 

Answer:

WeakSet is used to store a collection of weakly(weak references) held objects. The syntax would be as follows,
 

new WeakSet([iterable]);
Let's see the below example to explain it's behavior,
 
var ws = new WeakSet();
var user = {};
ws.add(user);
ws.has(user);    // true
ws.delete(user); // removes user from the set
ws.has(user);    // false, user has been removed
 

Answer:

The main difference is that references to objects in Set are strong while references to objects in WeakSet are weak. i.e, An object in WeakSet can be garbage collected if there is no other reference to it. Other differences are,

  1. Sets can store any value Whereas WeakSets can store only collections of objects
  2. WeakSet does not have size property unlike Set
  3. WeakSet does not have methods such as clear, keys, values, entries, forEach.
  4. WeakSet is not iterable.

Answer:

Below are the list of methods available on WeakSet,

  1. add(value): A new object is appended with the given value to the weakset
  2. delete(value): Deletes the value from the WeakSet collection.
  3. has(value): It returns true if the value is present in the WeakSet Collection, otherwise it returns false.
  4. length(): It returns the length of weakSetObject Let's see the functionality of all the above methods in an example,
var weakSetObject = new WeakSet();
var firstObject = {};
var secondObject = {};
// add(value)
weakSetObject.add(firstObject);
weakSetObject.add(secondObject);
console.log(weakSetObject.has(firstObject)); //true
console.log(weakSetObject.length()); //2
weakSetObject.delete(secondObject);
 

Answer:

The WeakMap object is a collection of key/value pairs in which the keys are weakly referenced. In this case, keys must be objects and the values can be arbitrary values. The syntax is looking like as below,
 

new WeakMap([iterable])
Let's see the below example to explain it's behavior,
 
var ws = new WeakMap();
 var user = {};
 ws.set(user);
 ws.has(user);    // true
 ws.delete(user); // removes user from the map
 ws.has(user);    // false, user has been removed
 

Answer:

The main difference is that references to key objects in Map are strong while references to key objects in WeakMap are weak. i.e, A key object in WeakMap can be garbage collected if there is no other reference to it. Other differences are,

  1. Maps can store any key type Whereas WeakMaps can store only collections of key objects
  2. WeakMap does not have size property unlike Map
  3. WeakMap does not have methods such as clear, keys, values, entries, forEach.
  4. WeakMap is not iterable.

Answer:

Below are the list of methods available on WeakMap,

  1. set(key, value): Sets the value for the key in the WeakMap object. Returns the WeakMap object.
  2. delete(key): Removes any value associated to the key.
  3. has(key): Returns a Boolean asserting whether a value has been associated to the key in the WeakMap object or not.
  4. get(key): Returns the value associated to the key, or undefined if there is none. Let's see the functionality of all the above methods in an example,
var weakMapObject = new WeakMap();
var firstObject = {};
var secondObject = {};
// set(key, value)
weakMapObject.set(firstObject, 'John');
weakMapObject.set(secondObject, 100);
console.log(weakMapObject.has(firstObject)); //true
console.log(weakMapObject.get(firstObject)); // John
weakMapObject.delete(secondObject);
 

Answer:

The uneval() is an inbuilt function which is used to create a string representation of the source code of an Object. It is a top-level function and is not associated with any object. Let's see the below example to know more about it's functionality,
 

var a = 1;
uneval(a); // returns a String containing 1
uneval(function user() {}); // returns "(function user(){})"
 

Answer:

The encodeURI() function is used to encode complete URI which has special characters except (, / ? : @ & = + $ #) characters.
 

var uri = 'https://mozilla.org/?x=?????';
var encoded = encodeURI(uri);
console.log(encoded); // https://mozilla.org/?x=%D1%88%D0%B5%D0%BB%D0%BB%D1%8B
 

Answer:

The decodeURI() function is used to decode a Uniform Resource Identifier (URI) previously created by encodeURI().
 

var uri = 'https://mozilla.org/?x=?????';
 var encoded = encodeURI(uri);
 console.log(encoded); // https://mozilla.org/?x=%D1%88%D0%B5%D0%BB%D0%BB%D1%8B
try {
  console.log(decodeURI(encoded)); // "https://mozilla.org/?x=?????"
} catch(e) { // catches a malformed URI
  console.error(e);
}
 

Answer:

The window object provided a print() method which is used to print the contents of the current window. It opens a Print dialog box which lets you choose between various printing options. Let's see the usage of print method in an example,
 

Note: In most browsers, it will block while the print dialog is open.

Answer:

The uneval function returns the source of a given object; whereas the eval function does the opposite, by evaluating that source code in a different memory area. Let's see an example to clarify the difference,
 

var msg = uneval(function greeting() { return 'Hello, Good morning'; });
var greeting = eval(msg);
greeting(); // returns "Hello, Good morning"
 

Answer:

An anonymous function is a function without a name! Anonymous functions are commonly assigned to a variable name or used as a callback function. The syntax would be as below,
 

function (optionalParameters) {
  //do something
}

const myFunction = function(){ //Anonymous function assigned to a variable
  //do something
};

[1, 2, 3].map(function(element){ //Anonymous function used as a callback function
  //do something
});
Let's see the above anonymous function in an example,
 
var x = function (a, b) {return a * b};
var z = x(5, 10);
console.log(z); // 50
 

Answer:

A local variable takes precedence over a global variable with the same name. Let's see this behavior in an example.
 

var msg = "Good morning";
function greeting() {
   msg = "Good Evening";
   console.log(msg);
}
greeting();
 

Answer:

ECMAScript 5 introduced javascript object accessors or computed properties through getters and setters. Getters uses the get keyword whereas Setters uses the set keyword.
 

var user = {
  firstName: "John",
  lastName : "Abraham",
  language : "en",
  get lang() {
    return this.language;
  }
  set lang(lang) {
  this.language = lang;
  }
};
console.log(user.lang); // getter access lang as en
user.lang = 'fr';
console.log(user.lang); // setter used to set lang as fr
 

Answer:

The Object.defineProperty() static method is used to define a new property directly on an object, or modify an existing property on an object, and returns the object. Let's see an example to know how to define property,
 

const newObject = {};

Object.defineProperty(newObject, 'newProperty', {
  value: 100,
  writable: false
});

console.log(newObject.newProperty); // 100

newObject.newProperty = 200; // It throws an error in strict mode due to writable setting
 

Answer:

Both have similar results until unless you use classes. If you use get the property will be defined on the prototype of the object whereas using Object.defineProperty() the property will be defined on the instance it is applied to.

Answer:

Below are the list of benefits of Getters and Setters,

  1. They provide simpler syntax
  2. They are used for defining computed properties, or accessors in JS.
  3. Useful to provide equivalence relation between properties and methods
  4. They can provide better data quality
  5. Useful for doing things behind the scenes with the encapsulated logic.

Answer:

Yes, You can use the Object.defineProperty() method to add Getters and Setters. For example, the below counter object uses increment, decrement, add and subtract properties,
 

var obj = {counter : 0};

// Define getters
Object.defineProperty(obj, "increment", {
  get : function () {this.counter++;}
});
Object.defineProperty(obj, "decrement", {
  get : function () {this.counter--;}
});

// Define setters
Object.defineProperty(obj, "add", {
  set : function (value) {this.counter += value;}
});
Object.defineProperty(obj, "subtract", {
  set : function (value) {this.counter -= value;}
});

obj.add = 10;
obj.subtract = 5;
console.log(obj.increment); //6
console.log(obj.decrement); //5
 

Answer:

The switch case statement in JavaScript is used for decision making purposes. In a few cases, using the switch case statement is going to be more convenient than if-else statements. The syntax would be as below,
 

switch (expression)
{
    case value1:
        statement1;
        break;
    case value2:
        statement2;
        break;
    .
    .
    case valueN:
        statementN;
        break;
    default:
        statementDefault;
}

The above multi-way branch statement provides an easy way to dispatch execution to different parts of code based on the value of the expression.

Answer:

Below are the list of conventions should be taken care,

  1. The expression can be of type either number or string.
  2. Duplicate values are not allowed for the expression.
  3. The default statement is optional. If the expression passed to switch does not match with any case value then the statement within default case will be executed.
  4. The break statement is used inside the switch to terminate a statement sequence.
  5. The break statement is optional. But if it is omitted, the execution will continue on into the next case.

Answer:

A primitive data type is data that has a primitive value (which has no properties or methods). There are 5 types of primitive data types.

  1. string
  2. number
  3. boolean
  4. null
  5. undefined

Answer:

There are 3 possible ways for accessing the property of an object.

1. Dot notation: It uses dot for accessing the properties

objectName.property

2. Square brackets notation: It uses square brackets for property access
objectName["property"]

3. Expression notation: It uses expression in the square brackets
objectName[expression]
 

Answer:

JavaScript functions follow below rules for parameters,

  1. The function definitions do not specify data types for parameters.
  2. Do not perform type checking on the passed arguments.
  3. Do not check the number of arguments received. i.e, The below function follows the above rules,
function functionName(parameter1, parameter2, parameter3) {
  console.log(parameter1); // 1
}
functionName(1);
 

Answer:

An error object is a built in error object that provides error information when an error occurs. It has two properties: name and message. For example, the below function logs error details,
 

try {
  greeting("Welcome");
}
catch(err) {
  console.log(err.name + "
" + err.message); }
 

Answer:

A SyntaxError is thrown if you try to evaluate code with a syntax error. For example, the below missing quote for the function parameter throws a syntax error
 

try {
  eval("greeting('welcome)");   // Missing ' will produce an error
}
catch(err) {
  console.log(err.name);
}
 

Answer:

There are 6 different types of error names returned from error object,
 

Error Name Description
EvalError An error has occurred in the eval() function
RangeError An error has occurred with a number "out of range"
ReferenceError An error due to an illegal reference
SyntaxError An error due to a syntax error
TypeError An error due to a type error
URIError An error due to encodeURI()

Answer:

Below are the list of statements used in an error handling,

  1. try: This statement is used to test a block of code for errors
  2. catch: This statement is used to handle the error
  3. throw: This statement is used to create custom errors.
  4. finally: This statement is used to execute code after try and catch regardless of the result. 

Answer:

  1. Entry Controlled loops: In this kind of loop type, the test condition is tested before entering the loop body. For example, For Loop and While Loop comes under this category.
  2. Exit Controlled Loops: In this kind of loop type, the test condition is tested or evaluated at the end of the loop body. i.e, the loop body will execute at least once irrespective of test condition true or false. For example, do-while loop comes under this category.

Answer:

Node.js is a server-side platform built on Chrome's JavaScript runtime for easily building fast and scalable network applications. It is an event-based, non-blocking, asynchronous I/O runtime that uses Google's V8 JavaScript engine and libuv library.

Answer:

The Intl object is the namespace for the ECMAScript Internationalization API, which provides language sensitive string comparison, number formatting, and date and time formatting. It provides access to several constructors and language sensitive functions.

Answer:

You can use the Intl.DateTimeFormat object which is a constructor for objects that enable language-sensitive date and time formatting. Let's see this behavior with an example,

var date = new Date(Date.UTC(2019, 07, 07, 3, 0, 0));
console.log(new Intl.DateTimeFormat('en-GB').format(date)); // 07/08/2019
console.log(new Intl.DateTimeFormat('en-AU').format(date)); // 07/08/2019
 

Answer:

An iterator is an object which defines a sequence and a return value upon its termination. It implements the Iterator protocol with a next() method which returns an object with two properties: value (the next value in the sequence) and done (which is true if the last value in the sequence has been consumed).

Answer:

Synchronous iteration was introduced in ES6 and it works with below set of components,

Iterable: It is an object which can be iterated over via a method whose key is Symbol.iterator. Iterator: It is an object returned by invoking [Symbol.iterator]() on an iterable. This iterator object wraps each iterated element in an object and returns it via next() method one by one. IteratorResult: It is an object returned by next() method. The object contains two properties; the value property contains an iterated element and the done property determines whether the element is the last element or not.

Let's demonstrate synchronous iteration with an array as below
 

const iterable = ['one', 'two', 'three'];
const iterator = iterable[Symbol.iterator]();
console.log(iterator.next());  // { value: 'one', done: false }
console.log(iterator.next());  // { value: 'two', done: false }
console.log(iterator.next());  // { value: 'three', done: false }
console.log(iterator.next());  // { value: 'undefined, done: true }

Answer:

The Event Loop is a queue of callback functions. When an async function executes, the callback function is pushed into the queue. The JavaScript engine doesn't start processing the event loop until the async function has finished executing the code. Note: It allows Node.js to perform non-blocking I/O operations even though JavaScript is single-threaded.

Answer:

Call Stack is a data structure for javascript interpreters to keep track of function calls in the program. It has two major actions,

  1. Whenever you call a function for its execution, you are pushing it to the stack.
  2. Whenever the execution is completed, the function is popped out of the stack.

Let's take an example and it's state representation in a diagram format
 

function hungry() {
   eatFruits();
}
function eatFruits() {
   return "I'm eating fruits";
}

// Invoke the `hungry` function
hungry();

he above code processed in a call stack as below,

  1. Add the hungry() function to the call stack list and execute the code.
  2. Add the eatFruits() function to the call stack list and execute the code.
  3. Delete the eatFruits() function from our call stack list.
  4. Delete the hungry() function from the call stack list since there are no items anymore.

Answer:

A decorator is an expression that evaluates to a function and that takes the target, name, and decorator descriptor as arguments. Also, it optionally returns a decorator descriptor to install on the target object. Let's define admin decorator for user class at design time,
 

function admin(isAdmin) {
   return function(target) {
       target.isAdmin = isAdmin;
   }
}

@admin(true)
class User() {
}
console.log(User.isAdmin); //true

 @admin(false)
 class User() {
 }
 console.log(User.isAdmin); //false
 

Answer:

Below are the list of properties available on Intl object,

  1. Collator: These are the objects that enable language-sensitive string comparison.
  2. DateTimeFormat: These are the objects that enable language-sensitive date and time formatting.
  3. ListFormat: These are the objects that enable language-sensitive list formatting.
  4. NumberFormat: Objects that enable language-sensitive number formatting.
  5. PluralRules: Objects that enable plural-sensitive formatting and language-specific rules for plurals.
  6. RelativeTimeFormat: Objects that enable language-sensitive relative time formatting.

Answer:

The unary(+) operator is used to convert a variable to a number.If the variable cannot be converted, it will still become a number but with the value NaN. Let's see this behavior in an action.

var x = "100";
var y = + x;
console.log(typeof x, typeof y); // string, number

var a = "Hello";
var b = + a;
console.log(typeof a, typeof b, b); // string, number, NaN
 

Answer:

The sort() method is used to sort the elements of an array in place and returns the sorted array. The example usage would be as below,

var months = ["Aug", "Sep", "Jan", "June"];
months.sort();
console.log(months); //  ["Aug", "Jan", "June", "Sep"]
 

Answer:

The compareFunction is used to define the sort order. If omitted, the array elements are converted to strings, then sorted according to each character's Unicode code point value. Let's take an example to see the usage of compareFunction,

let numbers = [1, 2, 5, 3, 4];
numbers.sort((a, b) => b - a);
console.log(numbers); // [5, 4, 3, 2, 1]
 

Answer:

You can use the reverse() method to reverse the elements in an array. This method is useful to sort an array in descending order. Let's see the usage of reverse() method in an example,

let numbers = [1, 2, 5, 3, 4];
numbers.sort((a, b) => b - a);
numbers.reverse();
console.log(numbers); // [1, 2, 3, 4 ,5]
 

Answer:

You can use Math.min and Math.max methods on array variables to find the minimum and maximum elements within an array. Let's create two functions to find the min and max value with in an array,

var marks = [50, 20, 70, 60, 45, 30];
function findMin(arr) {
  return Math.min.apply(null, arr);
}
function findMax(arr) {
  return Math.max.apply(null, arr);
}

console.log(findMin(marks));
console.log(findMax(marks));
 

Answer:

You can write functions which loop through an array comparing each value with the lowest value or highest value to find the min and max values. Let's create those functions to find min and max values,

var marks = [50, 20, 70, 60, 45, 30];
 function findMin(arr) {
   var length = arr.length
   var min = Infinity;
   while (length--) {
     if (arr[length] < min) {
       min = arr[len];
     }
   }
   return min;
 }

 function findMax(arr) {
   var length = arr.length
   var max = -Infinity;
   while (len--) {
     if (arr[length] > max) {
       max = arr[length];
     }
   }
   return max;
 }

 console.log(findMin(marks));
 console.log(findMax(marks));
 

Answer:

The empty statement is a semicolon (;) indicating that no statement will be executed, even if JavaScript syntax requires one. Since there is no action with an empty statement you might think that it's usage is quite less, but the empty statement is occasionally useful when you want to create a loop that has an empty body. For example, you can initialize an array with zero values as below,

// Initialize an array a
for(int i=0; i < a.length; a[i++] = 0) ;
 

Answer:

The comma operator is used to evaluate each of its operands from left to right and returns the value of the last operand. This is totally different from comma usage within arrays, objects, and function arguments and parameters. For example, the usage for numeric expressions would be as below,

var x = 1;
x = (x++, x);

console.log(x); // 2
 

Answer:

It is normally used to include multiple expressions in a location that requires a single expression. One of the common usages of this comma operator is to supply multiple parameters in a for loop. For example, the below for loop uses multiple expressions in a single location using comma operator,

for (var a = 0, b =10; a <= 10; a++, b--)
You can also use the comma operator in a return statement where it processes before returning.
function myFunction() {
   var a = 1;
   return (a += 10, a); // 11
}
 

Answer:

TypeScript is a typed superset of JavaScript created by Microsoft that adds optional types, classes, async/await, and many other features, and compiles to plain JavaScript. Angular built entirely in TypeScript and used as a primary language. You can install it globally as

npm install -g typescript
Let's see a simple example of TypeScript usage,
function greeting(name: string): string {
   return "Hello, " + name;
}

let user = "Sudheer";

console.log(greeting(user));
The greeting method allows only string type as argument.

Answer:

Below are the list of differences between javascript and typescript,

feature typescript javascript
Language paradigm Object oriented programming language Scripting language
Typing support Supports static typing It has dynamic typing
Modules Supported Not supported
Interface It has interfaces concept Doesn't support interfaces
Optional parameters Functions support optional parameters No support of optional parameters for functions

Answer:

Below are some of the advantages of typescript over javascript,

  1. TypeScript is able to find compile time errors at the development time only and it makes sures less runtime errors. Whereas javascript is an interpreted language.
  2. TypeScript is strongly-typed or supports static typing which allows for checking type correctness at compile time. This is not available in javascript.
  3. TypeScript compiler can compile the .ts files into ES3,ES4 and ES5 unlike ES6 features of javascript which may not be supported in some browsers.

Answer:

An object initializer is an expression that describes the initialization of an Object. The syntax for this expression is represented as a comma-delimited list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ({}). This is also known as literal notation. It is one of the ways to create an object.

var initObject = {a: 'John', b: 50, c: {}};

console.log(initObject.a); // John
 

Answer:

The constructor method is a special method for creating and initializing an object created within a class. If you do not specify a constructor method, a default constructor is used. The example usage of constructor would be as below,
 

class Employee {
  constructor() {
    this.name = "John";
  }
}

var employeeObject = new Employee();

console.log(employeeObject.name); // John
 

Answer:

The "constructor" in a class is a special method and it should be defined only once in a class. i.e, If you write a constructor method more than once in a class it will throw a SyntaxError error.
 

class Employee {
   constructor() {
     this.name = "John";
   }
   constructor() {   //  Uncaught SyntaxError: A class may only have one constructor
     this.age = 30;
   }
 }

 var employeeObject = new Employee();

 console.log(employeeObject.name);
 

Answer:

You can use the super keyword to call the constructor of a parent class. Remember that super() must be called before using 'this' reference. Otherwise it will cause a reference error. Let's the usage of it,
 

class Square extends Rectangle {
  constructor(length) {
    super(length, length);
    this.name = 'Square';
  }

  get area() {
    return this.width * this.height;
  }

  set area(value) {
    this.area = value;
  }
}
 

Answer:

You can use the Object.getPrototypeOf(obj) method to return the prototype of the specified object. i.e. The value of the internal prototype property. If there are no inherited properties then null value is returned.
 

const newPrototype = {};
const newObject = Object.create(newPrototype);

console.log(Object.getPrototypeOf(newObject) === newPrototype); // true
 

Answer:

In ES5, it will throw a TypeError exception if the obj parameter isn't an object. Whereas in ES2015, the parameter will be coerced to an Object.
 

// ES5
Object.getPrototypeOf('James'); // TypeError: "James" is not an object
// ES2015
Object.getPrototypeOf('James'); // String.prototype
 

Answer:

You can use the Object.setPrototypeOf() method that sets the prototype (i.e., the internal Prototype property) of a specified object to another object or null. For example, if you want to set prototype of a square object to rectangle object would be as follows,
 

Object.setPrototypeOf(Square.prototype, Rectangle.prototype);
Object.setPrototypeOf({}, null);
 

Answer:

The Object.isExtensible() method is used to determine if an object is extendable or not. i.e, Whether it can have new properties added to it or not.
 

const newObject = {};
console.log(Object.isExtensible(newObject)); //true
Note: By default, all the objects are extendable. i.e, The new properties can be added or modified.

Answer:

The Object.preventExtensions() method is used to prevent new properties from ever being added to an object. In other words, it prevents future extensions to the object. Let's see the usage of this property,

const newObject = {};
Object.preventExtensions(newObject); // NOT extendable

try {
  Object.defineProperty(newObject, 'newProperty', { // Adding new property
    value: 100
  });
} catch (e) {
  console.log(e); // TypeError: Cannot define property newProperty, object is not extensible
}
 

Answer:

You can mark an object non-extensible in 3 ways,

  1. Object.preventExtensions
  2. Object.seal
  3. Object.freeze
var newObject = {};

Object.preventExtensions(newObject); // Prevent objects are non-extensible
Object.isExtensible(newObject); // false

var sealedObject = Object.seal({}); // Sealed objects are non-extensible
Object.isExtensible(sealedObject); // false

var frozenObject = Object.freeze({}); // Frozen objects are non-extensible
Object.isExtensible(frozenObject); // false
 

Answer:

The Object.defineProperties() method is used to define new or modify existing properties directly on an object and returning the object. Let's define multiple properties on an empty object,

const newObject = {};

Object.defineProperties(newObject, {
  newProperty1: {
    value: 'John',
    writable: true
  },
  newProperty2: {}
});
 

Answer:

The MEAN (MongoDB, Express, AngularJS, and Node.js) stack is the most popular open-source JavaScript software tech stack available for building dynamic web apps where you can write both the server-side and client-side halves of the web project entirely in JavaScript.

Answer:

Obfuscation is the deliberate act of creating obfuscated javascript code(i.e, source or machine code) that is difficult for humans to understand. It is something similar to encryption, but a machine can understand the code and execute it. Let's see the below function before Obfuscation,

function greeting() {
   console.log('Hello, welcome to JS world');
}
And after the code Obfuscation, it would be appeared as below,
eval(function(p,a,c,k,e,d){e=function(c){return c};if(!''.replace(/^/,String)){while(c--){d[c]=k[c]||c}k=[function(e){return d[e]}];e=function(){return'\\w+'};c=1};while(c--){if(k[c]){p=p.replace(new RegExp('\\b'+e(c)+'\\b','g'),k[c])}}return p}('2 1(){0.3(\'4, 7 6 5 8\')}',9,9,'console|greeting|function|log|Hello|JS|to|welcome|world'.split('|'),0,{}))
 

Answer:

Below are the few reasons for Obfuscation,

  1. The Code size will be reduced. So data transfers between server and client will be fast.
  2. It hides the business logic from outside world and protects the code from others
  3. Reverse engineering is highly difficult
  4. The download time will be reduced

Answer:

Minification is the process of removing all unnecessary characters(empty spaces are removed) and variables will be renamed without changing it's functionality. It is also a type of obfuscation 

Answer:

Normally it is recommended to use minification for heavy traffic and intensive requirements of resources. It reduces file sizes with below benefits,

  1. Decreases loading times of a web page
  2. Saves bandwidth usages

Answer:

Below are the main differences between Obfuscation and Encryption,

Feature Obfuscation Encryption
Definition Changing the form of any data in any other form Changing the form of information to an unreadable format by using a key
A key to decode It can be decoded without any key It is required
Target data format It will be converted to a complex form Converted into an unreadable format

Answer:

There are many online/offline tools to minify the javascript files,

  1. Google's Closure Compiler
  2. UglifyJS2
  3. jsmin
  4. javascript-minifier.com/
  5. prettydiff.com

Answer:

JavaScript can be used to perform HTML form validation. For example, if the form field is empty, the function needs to notify, and return false, to prevent the form being submitted. Lets' perform user login in an html form,

<form name="myForm" onsubmit="return validateForm()" method="post">
User name: <input type="text" name="uname">
<input type="submit" value="Submit">
</form>
And the validation on user login is below,
function validateForm() {
  var x = document.forms["myForm"]["uname"].value;
  if (x == "") {
    alert("The username shouldn't be empty");
    return false;
  }
}
 

Answer:

You can perform HTML form validation automatically without using javascript. The validation enabled by applying the required attribute to prevent form submission when the input is empty.

<form method="post">
  <input type="text" name="uname" required>
  <input type="submit" value="Submit">
</form>
 

Answer:

The below DOM methods are available for constraint validation on an invalid input,

  1. checkValidity(): It returns true if an input element contains valid data.
  2. setCustomValidity(): It is used to set the validationMessage property of an input element. Let's take an user login form with DOM validations
function myFunction() {
  var userName = document.getElementById("uname");
  if (!userName.checkValidity()) {
    document.getElementById("message").innerHTML = userName.validationMessage;
  } else {
    document.getElementById("message").innerHTML = "Entered a valid username";
  }
}
 

Answer:

Below are the list of some of the constraint validation DOM properties available,

  1. validity: It provides a list of boolean properties related to the validity of an input element.
  2. validationMessage: It displays the message when the validity is false.
  3. willValidate: It indicates if an input element will be validated or not.

Answer:

The validity property of an input element provides a set of properties related to the validity of data.

  1. customError: It returns true, if a custom validity message is set.
  2. patternMismatch: It returns true, if an element's value does not match its pattern attribute.
  3. rangeOverflow: It returns true, if an element's value is greater than its max attribute.
  4. rangeUnderflow: It returns true, if an element's value is less than its min attribute.
  5. stepMismatch: It returns true, if an element's value is invalid according to step attribute.
  6. tooLong: It returns true, if an element's value exceeds its maxLength attribute.
  7. typeMismatch: It returns true, if an element's value is invalid according to type attribute.
  8. valueMissing: It returns true, if an element with a required attribute has no value.
  9. valid: It returns true, if an element's value is valid.

Answer:

You can use the Object.getOwnPropertyDescriptors() method which returns all own property descriptors of a given object. The example usage of this method is below,
 

const newObject = {
   a: 1,
   b: 2,
   c: 3
 };
const descriptorsObject = Object.getOwnPropertyDescriptors(newObject);
console.log(descriptorsObject.a.writable); //true
console.log(descriptorsObject.a.configurable); //true
console.log(descriptorsObject.a.enumerable); //true
console.log(descriptorsObject.a.value); // 1
 

Answer:

No, javascript does not natively support enums. But there are different kinds of solutions to simulate them even though they may not provide exact equivalents. For example, you can use freeze or seal on object,
 

var DaysEnum = Object.freeze({"monday":1, "tuesday":2, "wednesday":3, ...})
 

Answer:

An enum is a type restricting variables to one value from a predefined set of constants. JavaScript has no enums but typescript provides built-in enum support.
 

enum Color {
 RED, GREEN, BLUE
}
 

Answer:

You can use the Object.getOwnPropertyNames() method which returns an array of all properties found directly in a given object. Let's the usage of it in an example,

const newObject = {
  a: 1,
  b: 2,
  c: 3
};

console.log(Object.getOwnPropertyNames(newObject));  ["a", "b", "c"]
 

Answer:

A property descriptor is a record which has the following attributes

  1. value: The value associated with the property
  2. writable: Determines whether the value associated with the property can be changed or not
  3. configurable: Returns true if the type of this property descriptor can be changed and if the property can be deleted from the corresponding object.
  4. enumerable: Determines whether the property appears during enumeration of the properties on the corresponding object or not.
  5. set: A function which serves as a setter for the property
  6. get: A function which serves as a getter for the property

Answer:

The extends keyword is used in class declarations/expressions to create a class which is a child of another class. It can be used to subclass custom classes as well as built-in objects. The syntax would be as below,
 

class ChildClass extends ParentClass { ... }
Let's take an example of Square subclass from Polygon parent class,
class Square extends Rectangle {
   constructor(length) {
     super(length, length);
     this.name = 'Square';
   }

   get area() {
     return this.width * this.height;
   }

   set area(value) {
     this.area = value;
   }
 }
 

Answer:

The window.location.url property will be helpful to modify the url but it reloads the page. HTML5 introduced the history.pushState() and history.replaceState() methods, which allow you to add and modify history entries, respectively. For example, you can use pushState as below,
 

window.history.pushState('page2', 'Title', '/page2.html');
 

Answer:

The Array#includes() method is used to determine whether an array includes a particular value among its entries by returning either true or false. Let's see an example to find an element(numeric and string) within an array.
 

var numericArray = [1, 2, 3, 4];
console.log(numericArray.includes(3)); // true

var stringArray = ['green', 'yellow', 'blue'];
console.log(stringArray.includes('blue')); //true
 

Answer:

You can use length and every method of arrays to compare two scalar(compared directly using ===) arrays. The combination of these expressions can give the expected result,
 

const arrayFirst = [1,2,3,4,5];
const arraySecond = [1,2,3,4,5];
console.log(arrayFirst.length === arraySecond.length && arrayFirst.every((value, index) => value === arraySecond[index])); // true
If you would like to compare arrays irrespective of order then you should sort them before,
const arrayFirst = [2,3,1,4,5];
const arraySecond = [1,2,3,4,5];
console.log(arrayFirst.length === arraySecond.length && arrayFirst.sort().every((value, index) => value === arraySecond[index])); //true
 

Answer:

The new URL() object accepts the url string and searchParams property of this object can be used to access the get parameters. Remember that you may need to use polyfill or window.location to access the URL in older browsers(including IE).
 

let urlString = "http://www.some-domain.com/about.html?x=1&y=2&z=3"; //window.location.href
let url = new URL(urlString);
let parameterZ = url.searchParams.get("z");
console.log(parameterZ); // 3
 

Answer:

You can use the Number.prototype.toLocaleString() method which returns a string with a language-sensitive representation such as thousand separator,currency etc of this number.
 

function convertToThousandFormat(x){
  return x.toLocaleString(); // 12,345.679
}

console.log(convertToThousandFormat(12345.6789));
 

Answer:

Both are totally unrelated programming languages and no relation between them. Java is statically typed, compiled, runs on its own VM. Whereas Javascript is dynamically typed, interpreted, and runs in a browser and nodejs environments. Let's see the major differences in a tabular format,
 

Feature Java JavaScript
Typed
It's a strongly typed language
It's a dynamic typed language
Paradigm Object oriented programming Prototype based programming
Scoping Block scoped Function-scoped
Concurrency
Thread based event based
Memory Uses more memory Uses less memory. Hence it will be used for web pages

Answer:

JavaScript doesn’t support namespace by default. So if you create any element(function, method, object, variable) then it becomes global and pollutes the global namespace. Let's take an example of defining two functions without any namespace,
 

function func1() {
    console.log("This is a first definition");

}
function func1() {
    console.log("This is a second definition");
}
func1(); // This is a second definition
 

Answer:

Even though JavaScript lacks namespaces, we can use Objects , IIFE to create namespaces.

1. Using Object Literal Notation: Let's wrap variables and functions inside an Object literal which acts as a namespace. After that you can access them using object notation
 

var namespaceOne = {
   function func1() {
       console.log("This is a first definition");
   }
}
var namespaceTwo = {
     function func1() {
         console.log("This is a second definition");
     }
 }
namespaceOne.func1(); // This is a first definition
namespaceTwo.func1(); // This is a second definition

2. Using IIFE (Immediately invoked function expression): The outer pair of parentheses of IIFE creates a local scope for all the code inside of it and makes the anonymous function a function expression. Due to that, you can create the same function in two different function expressions to act as a namespace.
 
(function() {
 function fun1(){
   console.log("This is a first definition");
   } fun1();
}());

(function() {
    function fun1(){
       console.log("This is a second definition");
   } fun1();
 }());

3. Using a block and a let/const declaration: In ECMAScript 6, you can simply use a block and a let declaration to restrict the scope of a variable to a block.
 
{
  let myFunction= function fun1(){
  console.log("This is a first definition");
  }
  myFunction();
 }
  //myFunction(): ReferenceError: myFunction is not defined.

 {
  let myFunction= function fun1(){
  console.log("This is a second definition");
  }
  myFunction();
 }
  //myFunction(): ReferenceError: myFunction is not defined.
 

Answer:

Initially iFrame needs to be accessed using either document.getElementBy or window.frames. After that contentWindow property of iFrame gives the access for targetFunction
 

document.getElementById('targetFrame').contentWindow.targetFunction();
window.frames[0].frameElement.contentWindow.targetFunction(); // Accessing iframe this way may not work in latest versions chrome and firefox
 

Answer:

You can use the getTimezoneOffset method of the date object. This method returns the time zone difference, in minutes, from current locale (host system settings) to UTC
 

var offset = new Date().getTimezoneOffset();
console.log(offset); // -480
 

Answer:

You can create both link and script elements in the DOM and append them as child to head tag. Let's create a function to add script and style resources as below,
 

function loadAssets(filename, filetype) {
  if (filetype == "css") { // External CSS file
       var fileReference = document.createElement("link")
       fileReference.setAttribute("rel", "stylesheet");
       fileReference.setAttribute("type", "text/css");
       fileReference.setAttribute("href", filename);
  } else if (filetype == "js") { // External JavaScript file
       var fileReference = document.createElement('script');
       fileReference.setAttribute("type", "text/javascript");
       fileReference.setAttribute("src", filename);
  }
  if (typeof fileReference != "undefined")
       document.getElementsByTagName("head")[0].appendChild(fileReference)
 }
 

Answer:

If you want to access any element in an HTML page, you need to start with accessing the document object. Later you can use any of the below methods to find the HTML element,

  1. document.getElementById(id): It finds an element by Id
  2. document.getElementsByTagName(name): It finds an element by tag name
  3. document.getElementsByClassName(name): It finds an element by class name

Answer:

jQuery is a popular cross-browser JavaScript library that provides Document Object Model (DOM) traversal, event handling, animations and AJAX interactions by minimizing the discrepancies across browsers. It is widely famous with its philosophy of “Write less, do more”. For example, you can display welcome message on the page load using jQuery as below,
 

$(document).ready(function(){ // It selects the document and apply the function on page load
    alert('Welcome to jQuery world');
});
Note: You can download it from jquery's official site or install it from CDNs, like google.

Answer:

V8 is an open source high-performance JavaScript engine used by the Google Chrome browser, written in C++. It is also being used in the node.js project. It implements ECMAScript and WebAssembly, and runs on Windows 7 or later, macOS 10.12+, and Linux systems that use x64, IA-32, ARM, or MIPS processors. Note: It can run standalone, or can be embedded into any C++ application.

Answer:

JavaScript is a loosely typed or a dynamic language because variables in JavaScript are not directly associated with any particular value type, and any variable can be assigned/reassigned with values of all types.
 

let age = 50;    // age is a number now
age  = 'old'; // age is a string now
age  = true;  // age is a boolean
 

Answer:

The void operator evaluates the given expression and then returns undefined(i.e, without returning value). The syntax would be as below,
 

void (expression)
void expression

Let's display a message without any redirection or reload
 
<a href="javascript:void(alert('Welcome to JS world'))">Click here to see a message</a>
Note: This operator is often used to obtain the undefined primitive value, using "void(0)".

Answer:

The cursor can be set to wait in JavaScript by using the property "cursor". Let's perform this behavior on page load using the below function.
 

function myFunction() {
window.document.body.style.cursor = "wait";
}
and this function invoked on page load
 
<body onload="myFunction()">
 

Answer:

You can create infinite loops using for and while loops without using any expressions. The for loop construct or syntax is better approach in terms of ESLint and code optimizer tools,
 

for (;;) {}
while(true) {
}
 

Answer:

JavaScript's with statement was intended to provide a shorthand for writing recurring accesses to objects. So it can help reduce file size by reducing the need to repeat a lengthy object reference without performance penalty. Let's take an example where it is used to avoid redundancy when accessing an object several times.
 

a.b.c.greeting   = 'welcome';
a.b.c.age = 32;
Using with it turns this into:
 
with(a.b.c) {
        greeting   = "welcome";
        age = 32;
}
But this with statement creates performance problems since one cannot predict whether an argument will refer to a real variable or to a property inside the with argument.

Answer:

Below are the list of some new features of ES6,

  1. Support for constants or immutable variables
  2. Block-scope support for variables, constants and functions
  3. Arrow functions
  4. Default parameters
  5. Rest and Spread Parameters
  6. Template Literals
  7. Multi-line Strings
  8. Destructuring Assignment
  9. Enhanced Object Literals
  10. Promises
  11. Classes
  12. Modules

Answer:

ES6 is the sixth edition of the javascript language and it was released in June 2015. It was initially known as ECMAScript 6 (ES6) and later renamed to ECMAScript 2015. Almost all the modern browsers support ES6 but for the old browsers there are many transpilers, like Babel.js etc.

Answer:

No, you cannot redeclare let and const variables. If you do, it throws below error

Uncaught SyntaxError: Identifier 'someVariable' has already been declared

Explanation: The variable declaration with var keyword refers to a function scope and the variable is treated as if it were declared at the top of the enclosing scope due to hoisting feature. So all the multiple declarations contributing to the same hoisted variable without any error. Let's take an example of re-declaring variables in the same scope for both var and let/const variables.
 
var name = 'John';
function myFunc() {
    var name = 'Nick';
    var name = 'Abraham'; // Re-assigned in the same function block
    alert(name); // Abraham
}
myFunc();
alert(name); // John

The block-scoped multi-declaration throws syntax error,
 
let name = 'John';
function myFunc() {
    let name = 'Nick';
    let name = 'Abraham'; // Uncaught SyntaxError: Identifier 'name' has already been declared
    alert(name);
}

myFunc();
alert(name);
 

Answer:

No, the const variable doesn't make the value immutable. But it disallows subsequent assignments(i.e, You can declare with assignment but can't assign another value later)
 

const userList = [];
userList.push('John'); // Can mutate even though it can't re-assign
console.log(userList); // ['John']
 

Answer:

In E5, we need to depend on logical OR operators to handle default values of function parameters. Whereas in ES6, Default function parameters feature allows parameters to be initialized with default values if no value or undefined is passed. Let's compare the behavior with an examples,
 

//ES5
var calculateArea = function(height, width) {
   height =  height || 50;
   width = width || 60;

   return width * height;
}
console.log(calculateArea()); //300

The default parameters makes the initialization more simpler,
 
//ES6
var calculateArea = function(height = 50, width = 60) {
   return width * height;
}

console.log(calculateArea()); //300
 

Answer:

Template literals or template strings are string literals allowing embedded expressions. These are enclosed by the back-tick (`) character instead of double or single quotes. In E6, this feature enables using dynamic expressions as below,
 

var greeting = `Welcome to JS World, Mr. ${firstName} ${lastName}.`

In ES5, you need break string like below,
 
var greeting = 'Welcome to JS World, Mr. ' + firstName + ' ' + lastName.`
Note: You can use multi-line strings and string interpolation features with template literals.

Answer:

In ES5, you would have to use newline escape characters('\n') and concatenation symbols(+) in order to get multi-line strings.
 

console.log('This is string sentence 1\n' +
'This is string sentence 2');
Whereas in ES6, You don't need to mention any newline sequence character,
 
console.log(`This is string sentence
'This is string sentence 2`);
 

Answer:

The nesting template is a feature supported within template literals syntax to allow inner backticks inside a placeholder ${ } within the template. For example, the below nesting template is used to display the icons based on user permissions whereas outer template checks for platform type,
 

const iconStyles = `icon ${ isMobilePlatform() ? '' :
 `icon-${user.isAuthorized ? 'submit' : 'disabled'}` }`;

You can write the above use case without nesting template features as well. However, the nesting template feature is more compact and readable.
 
//Without nesting templates
 const iconStyles = `icon ${ isMobilePlatform() ? '' :
  (user.isAuthorized ? 'icon-submit' : 'icon-disabled'}`;
 

Answer:

Tagged templates are the advanced form of templates in which tags allow you to parse template literals with a function. The tag function accepts the first parameter as an array of strings and remaining parameters as expressions. This function can also return manipulated strings based on parameters. Let's see the usage of this tagged template behavior of an IT professional skill set in an organization,
 

var user1 = 'John';
var skill1 = 'JavaScript';
var experience1 = 15;

var user2 = 'Kane';
var skill2 = 'JavaScript';
var experience2 = 5;

function myInfoTag(strings, userExp, experienceExp, skillExp) {
  var str0 = strings[0]; // "Mr/Ms. "
  var str1 = strings[1]; // " is a/an "
  var str2 = strings[2]; // "in"

  var expertiseStr;
  if (experienceExp > 10){
    expertiseStr = 'expert developer';
  } else if(skillExp > 5 && skillExp <= 10) {
    expertiseStr = 'senior developer';
  } else {
    expertiseStr = 'junior developer';
  }

  return ${str0}${userExp}${str1}${expertiseStr}${str2}${skillExp};
}

var output1 = myInfoTag`Mr/Ms. ${ user1 } is a/an ${ experience1 } in ${skill1}`;
var output2 = myInfoTag`Mr/Ms. ${ user2 } is a/an ${ experience2 } in ${skill2}`;

console.log(output1);// Mr/Ms. John is a/an expert developer in JavaScript
console.log(output2);// Mr/Ms. Kane is a/an junior developer in JavaScript
 

Answer:

ES6 provides a raw strings feature using the String.raw() method which is used to get the raw string form of template strings. This feature allows you to access the raw strings as they were entered, without processing escape sequences. For example, the usage would be as below,
 

var calculationString = String.raw `The sum of numbers is \n${1+2+3+4}!`;
console.log(calculationString); // The sum of numbers is 10
If you don't use raw strings, the newline character sequence will be processed by displaying the output in multiple lines
 
var calculationString = `The sum of numbers is \n${1+2+3+4}!`;
console.log(calculationString);
// The sum of numbers is
// 10

Also, the raw property is available on the first argument to the tag function
 
function tag(strings) {
   console.log(strings.raw[0]);
}
 

Answer:

The destructuring assignment is a JavaScript expression that makes it possible to unpack values from arrays or properties from objects into distinct variables. Let's get the month values from an array using destructuring assignment
 

var [one, two, three] = ['JAN', 'FEB', 'MARCH'];

console.log(one); // "JAN"
console.log(two); // "FEB"
console.log(three); // "MARCH"

and you can get user properties of an object using destructuring assignment,
 
var {name, age} = {name: 'John', age: 32};

console.log(name); // John
console.log(age); // 32
 

Answer:

A variable can be assigned a default value when the value unpacked from the array or object is undefined during destructuring assignment. It helps to avoid setting default values separately for each assignment. Let's take an example for both arrays and object use cases,
Arrays destructuring:
 

var x, y, z;

[x=2, y=4, z=6] = [10];
console.log(x); // 10
console.log(y); // 4
console.log(z); // 6

Objects destructuring:
 
var {x=2, y=4, z=6} = {x: 10};

console.log(x); // 10
console.log(y); // 4
console.log(z); // 6
 

Answer:

If you don't use destructuring assignment, swapping two values requires a temporary variable. Whereas using a destructuring feature, two variable values can be swapped in one destructuring expression. Let's swap two number variables in array destructuring assignment,
 

var x = 10, y = 20;

[x, y] = [y, x];
console.log(x); // 20
console.log(y); // 10
 

Answer:

Object literals make it easy to quickly create objects with properties inside the curly braces. For example, it provides shorter syntax for common object property definition as below.
 

//ES6
var x = 10, y = 20
obj = { x, y }
console.log(obj); // {x: 10, y:20}
//ES5
var x = 10, y = 20
obj = { x : x, y : y}
console.log(obj); // {x: 10, y:20}
 

Answer:

The dynamic imports using import() function syntax allows us to load modules on demand by using promises or the async/await syntax. Currently this feature is in stage4 proposal. The main advantage of dynamic imports is reduction of our bundle's sizes, the size/payload response of our requests and overall improvements in the user experience. The syntax of dynamic imports would be as below,
 

import('./Module').then(Module => Module.method());
 

Answer:

Below are some of the use cases of using dynamic imports over static imports,

1. Import a module on-demand or conditionally. For example, if you want to load a polyfill on legacy browser

if (isLegacyBrowser()) {
    import(···)
    .then(···);
}

2. Compute the module specifier at runtime. For example, you can use it for internationalization.
 
import(`messages_${getLocale()}.js`).then(···);
Import a module from within a regular script instead a module.

Answer:

Typed arrays are array-like objects from ECMAScript 6 API for handling binary data. JavaScript provides 8 Typed array types,

  1. Int8Array: An array of 8-bit signed integers
  2. Int16Array: An array of 16-bit signed integers
  3. Int32Array: An array of 32-bit signed integers
  4. Uint8Array: An array of 8-bit unsigned integers
  5. Uint16Array: An array of 16-bit unsigned integers
  6. Uint32Array: An array of 32-bit unsigned integers
  7. Float32Array: An array of 32-bit floating point numbers
  8. Float64Array: An array of 64-bit floating point numbers

For example, you can create an array of 8-bit signed integers as below

 

const a = new Int8Array();
// You can pre-allocate n bytes
const bytes = 1024
const a = new Int8Array(bytes)

Answer:

The module loaders provides the below features,

  1. Dynamic loading
  2. State isolation
  3. Global namespace isolation
  4. Compilation hooks
  5. Nested virtualization

Answer:

Collation is used for sorting a set of strings and searching within a set of strings. It is parameterized by locale and aware of Unicode. Let's take comparison and sorting features,

1. Comparison:
 

var list = [ "ä", "a", "z" ]; // In German,  "ä" sorts with "a" Whereas in Swedish, "ä" sorts after "z"
var l10nDE = new Intl.Collator("de");
var l10nSV = new Intl.Collator("sv");
console.log(l10nDE.compare("ä", "z") === -1); // true
console.log(l10nSV.compare("ä", "z") === +1); // true

2. Sorting:
 
var list = [ "ä", "a", "z" ]; // In German,  "ä" sorts with "a" Whereas in Swedish, "ä" sorts after "z"
var l10nDE = new Intl.Collator("de");
var l10nSV = new Intl.Collator("sv");
console.log(list.sort(l10nDE.compare)) // [ "a", "ä", "z" ]
console.log(list.sort(l10nSV.compare)) // [ "a", "z", "ä" ]
 

Answer:

The for...of statement creates a loop iterating over iterable objects or elements such as built-in String, Array, Array-like objects (like arguments or NodeList), TypedArray, Map, Set, and user-defined iterables. The basic usage of for...of statement on arrays would be as below,
 

let arrayIterable = [10, 20, 30, 40, 50];

for (let value of arrayIterable) {
  value ++;
  console.log(value); // 11 21 31 41 51
}
 

Answer:

Yes, postMessages can be considered very secure as long as the programmer/developer is careful about checking the origin and source of an arriving message. But if you try to send/receive a message without verifying its source will create cross-site scripting attacks.

Answer:

The second argument of postMessage method specifies which origin is allowed to receive the message. If you use the wildcard “*” as an argument then any origin is allowed to receive the message. In this case, there is no way for the sender window to know if the target window is at the target origin when sending the message. If the target window has been navigated to another origin, the other origin would receive the data. Hence, this may lead to XSS vulnerabilities.
 

targetWindow.postMessage(message, '*');
 

Answer:

Since the listener listens for any message, an attacker can trick the application by sending a message from the attacker’s origin, which gives an impression that the receiver received the message from the actual sender’s window. You can avoid this issue by validating the origin of the message on the receiver's end using the “message.origin” attribute. For examples, let's check the sender's origin http://www.some-sender.com on receiver side www.some-receiver.com,
 

//Listener on http://www.some-receiver.com/
window.addEventListener("message", function(message){
    if(/^http://www\.some-sender\.com$/.test(message.origin)){
         console.log('You received the data from valid sender', message.data);
   }
});
 

Answer:

You cannot avoid using postMessages completely(or 100%). Even though your application doesn’t use postMessage considering the risks, a lot of third party scripts use postMessage to communicate with the third party service. So your application might be using postMessage without your knowledge.

Answer:

The postMessages are synchronous in IE8 browser but they are asynchronous in IE9 and all other modern browsers (i.e, IE9+, Firefox, Chrome, Safari).Due to this asynchronous behaviour, we use a callback mechanism when the postMessage is returned.

Answer:

JavaScript is a multi-paradigm language, supporting imperative/procedural programming, Object-Oriented Programming and functional programming. JavaScript supports Object-Oriented Programming with prototypical inheritance.

Answer:

Internal JavaScript: It is the source code within the script tag. 
External JavaScript: The source code is stored in an external file(stored with .js extension) and referred with in the tag.

Answer:

Yes, JavaScript is faster than server side script. Because JavaScript is a client-side script it does not require any web server’s help for its computation or calculation. So JavaScript is always faster than any server-side script like ASP, PHP, etc.

Answer:

You can apply the checked property on the selected checkbox in the DOM. If the value is True means the checkbox is checked otherwise it is unchecked. For example, the below HTML checkbox element can be access using javascript as below,
 

<input type="checkbox" name="checkboxname" value="Agree"> Agree the conditions<br>
console.log(document.getElementById(‘checkboxname’).checked); // true or false
 

Answer:

The double tilde operator(~~) is known as double NOT bitwise operator. This operator is going to be a quicker substitute for Math.floor().

Answer:

You can use the String.prototype.charCodeAt() method to convert string characters to ASCII numbers. For example, let's find ASCII code for the first letter of 'ABC' string,
 

"ABC".charCodeAt(0) // returns 65

Whereas String.fromCharCode() method converts numbers to equal ASCII characters.
 
String.fromCharCode(65,66,67); // returns 'ABC'
 

Answer:

An ArrayBuffer object is used to represent a generic, fixed-length raw binary data buffer. You can create it as below,
 

let buffer = new ArrayBuffer(16); // create a buffer of length 16
alert(buffer.byteLength); // 16
To manipulate an ArrayBuffer, we need to use a “view” object.
 
//Create a DataView referring to the buffer
 let view = new DataView(buffer);
 

Answer:

The Error constructor creates an error object and the instances of error objects are thrown when runtime errors occur. The Error object can also be used as a base object for user-defined exceptions. The syntax of error object would be as below,
 

new Error([message[, fileName[, lineNumber]]])
You can throw user defined exceptions or errors using Error object in try...catch block as below,
 
try {
  if(withdraw > balance)
  throw new Error("Oops! You don't have enough balance");
} catch (e) {
  console.log(e.name + ': ' + e.message);
}
 

Answer:

The EvalError object indicates an error regarding the global eval() function. Even though this exception is not thrown by JavaScript anymore, the EvalError object remains for compatibility. The syntax of this expression would be as below,
 

new EvalError([message[, fileName[, lineNumber]]])
You can throw EvalError with in try...catch block as below,
 
try {
  throw new EvalError('Eval function error', 'someFile.js', 100);
} catch (e) {
  console.log(e.message, e.name, e.fileName);              // "Eval function error", "EvalError", "someFile.js"
 

Answer:

When you apply 'use strict'; syntax, some of the below cases will throw a SyntaxError before executing the script
1. When you use Octal syntax

var n = 022;
  1. Using with statement
  2. When you use delete operator on a variable name
  3. Using eval or arguments as variable or function argument name
  4. When you use newly reserved keywords
  5. When you declare a function in a block
if (someCondition) { function f() {} }

Hence, the errors from above cases are helpful to avoid errors in development/production environments.

Answer:

No. All objects have prototypes except for the base object which is created by the user, or an object that is created using the new keyword.

Answer:

Parameter is the variable name of a function definition whereas an argument represents the value given to a function when it is invoked. Let's explain this with a simple function
 

function myFunction(parameter1, parameter2, parameter3) {
  console.log(arguments[0]) // "argument1"
  console.log(arguments[1]) // "argument2"
  console.log(arguments[2]) // "argument3"
}
myFunction("argument1", "argument2", "argument3")
 

Answer:

The some() method is used to test whether at least one element in the array passes the test implemented by the provided function. The method returns a boolean value. Let's take an example to test for any odd elements,
 

var array = [1, 2, 3, 4, 5, 6 ,7, 8, 9, 10];

var odd = element ==> element % 2 !== 0;

console.log(array.some(odd)); // true (the odd element exists)
 

Answer:

The concat() method is used to join two or more arrays by returning a new array containing all the elements. The syntax would be as below,
 

array1.concat(array2, array3, ..., arrayX)

Let's take an example of array's concatenation with veggies and fruits arrays,
 
var veggies = ["Tomato", "Carrot", "Cabbage"];
  var fruits = ["Apple", "Orange", "Pears"];
  var veggiesAndFruits = veggies.concat(fruits);
  console.log(veggiesAndFruits); // Tomato, Carrot, Cabbage, Apple, Orange, Pears
 

Answer:

There are two ways to copy an object,
Shallow Copy: Shallow copy is a bitwise copy of an object. A new object is created that has an exact copy of the values in the original object. If any of the fields of the object are references to other objects, just the reference addresses are copied i.e., only the memory address is copied.
 

var empDetails = {
  name: "John", age: 25, expertise: "Software Developer"
}

to create a duplicate
 
var empDetailsShallowCopy = empDetails    //Shallow copying!

if we change some property value in the duplicate one like this:
 
empDetailsShallowCopy.name = "Johnson"
 

The above statement will also change the name of empDetails, since we have a shallow copy. That means we're losing the original data as well.

Deep copy: A deep copy copies all fields, and makes copies of dynamically allocated memory pointed to by the fields. A deep copy occurs when an object is copied along with the objects to which it refers.

 

var empDetails = {
  name: "John", age: 25, expertise: "Software Developer"
}

Create a deep copy by using the properties from the original object into new variable
 
var empDetailsDeepCopy = {
  name: empDetails.name,
  age: empDetails.age,
  expertise: empDetails.expertise
}

Now if you change empDetailsDeepCopy.name, it will only affect empDetailsDeepCopy & not empDetails

Answer:

The repeat() method is used to construct and return a new string which contains the specified number of copies of the string on which it was called, concatenated together. Remember that this method has been added to the ECMAScript 2015 specification. Let's take an example of Hello string to repeat it 4 times,
 

'Hello'.repeat(4);  // 'HelloHelloHelloHello'
 

Answer:

The matchAll() method can be used to return an iterator of all results matching a string against a regular expression. For example, the below example returns an array of matching string results against a regular expression,
 

let regexp = /Hello(\d?))/g;
let greeting = 'Hello1Hello2Hello3';

let greetingList = [...greeting.matchAll(regexp)];

console.log(greetingList[0]); //Hello1
console.log(greetingList[1]); //Hello2
console.log(greetingList[2]); //Hello3
 

Answer:

The trim method of string prototype is used to trim on both sides of a string. But if you want to trim especially at the beginning or ending of the string then you can use trimStart/trimLeft and trimEnd/trimRight methods. Let's see an example of these methods on a greeting message,
 

var greeting = '   Hello, Goodmorning!   ';

console.log(greeting); // "   Hello, Goodmorning!   "
console.log(greeting.trimStart()); // "Hello, Goodmorning!   "
console.log(greeting.trimLeft()); // "Hello, Goodmorning!   "

console.log(greeting.trimEnd()); // "   Hello, Goodmorning!"
console.log(greeting.trimRight()); // "   Hello, Goodmorning!"
 

Answer:

Let's take console statement with unary operator as given below,
 

console.log(+ 'Hello');

The output of the above console log statement returns NaN. Because the element is prefixed by the unary operator and the JavaScript interpreter will try to convert that element into a number type. Since the conversion fails, the value of the statement results in NaN value.

Answer:

A thunk is just a function which delays the evaluation of the value. It doesn’t take any arguments but gives the value whenever you invoke the thunk. i.e, It is used not to execute now but it will be sometime in the future. Let's take a synchronous example,
 

const add = (x,y) => x + y;

const thunk = () => add(2,3);

thunk() // 5
 

Answer:

The asynchronous thunks are useful to make network requests. Let's see an example of network requests,
 

function fetchData(fn){
  fetch('https://jsonplaceholder.typicode.com/todos/1')
  .then(response => response.json())
  .then(json => fn(json))
}

const asyncThunk = function (){
   return fetchData(function getData(data){
      console.log(data)
  })
}

asyncThunk()

The getData function won't be called immediately but it will be invoked only when the data is available from API endpoint. The setTimeout function is also used to make our code asynchronous. The best real time example is redux state management library which uses the asynchronous thunks to delay the actions to dispatch.

Answer:

The easiest approach is using regular expressions to detect and replace newlines in the string. In this case, we use replace function along with string to replace with, which in our case is an empty string.
 

function remove_linebreaks( var message ) {
    return message.replace( /[\r\n]+/gm, "" );
}
In the above expression, g and m are for global and multiline flags.

Answer:

repaint occurs when changes are made which affect the visibility of an element, but not its layout. Examples of this include outline, visibility, or background color. A reflow involves changes that affect the layout of a portion of the page (or the whole page). Resizing the browser window, changing the font, content changing (such as user typing text), using JavaScript methods involving computed styles, adding or removing elements from the DOM, and changing an element's classes are a few of the things that can trigger reflow. Reflow of an element causes the subsequent reflow of all child and ancestor elements as well as any elements following it in the DOM.

Answer:

Negating an array with ! character will coerce the array into a boolean. Since Arrays are considered to be truthy So negating it will return false.
 

console.log(![]); // false
 

Answer:

If you add two arrays together, it will convert them both to strings and concatenate them. For example, the result of adding arrays would be as below,
 

console.log(['a'] + ['b']);  // "ab"
console.log([] + []); // ""
console.log(![] + []); // "false", because ![] returns false.
 

Answer:

If you prepend the additive(+) operator on falsy values(null, undefined, NaN, false, ""), the falsy value converts to a number value zero. Let's display them on browser console as below,
 

console.log(+null); // 0
console.log(+undefined);// NaN
console.log(+false); // 0
console.log(+NaN); // NaN
console.log(+""); // 0
 

Answer:

The self string can be formed with the combination of []()!+ characters. You need to remember the below conventions to achieve this pattern.

  1. Since Arrays are truthful values, negating the arrays will produce false: ![] === false
  2. As per JavaScript coercion rules, the addition of arrays together will toString them: [] + [] === ""
  3. Prepend an array with + operator will convert an array to false, the negation will make it true and finally converting the result will produce value '1': +(!(+[])) === 1

By applying the above rules, we can derive below conditions

 

![] + [] === "false"
+!+[] === 1

Now the character pattern would be created as below, 
 
       s               e               l               f
 ^^^^^^^^^^^^^   ^^^^^^^^^^^^^   ^^^^^^^^^^^^^   ^^^^^^^^^^^^^

 (![] + [])[3] + (![] + [])[4] + (![] + [])[2] + (![] + [])[0]
 ^^^^^^^^^^^^^   ^^^^^^^^^^^^^   ^^^^^^^^^^^^^   ^^^^^^^^^^^^^
(![] + [])[+!+[]+!+[]+!+[]] +
(![] + [])[+!+[]+!+[]+!+[]+!+[]] +
(![] + [])[+!+[]+!+[]] +
(![] + [])[+[]]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(![]+[])[+!+[]+!+[]+!+[]]+(![]+[])[+!+[]+!+[]+!+[]+!+[]]+(![]+[])[+!+[]+!+[]]+(![]+[])[+[]]
 

Answer:

You can apply the filter method on the array by passing Boolean as a parameter. This way it removes all falsy values(0, undefined, null, false and "") from the array.
 

const myArray = [false, null, 1,5, undefined]
myArray.filter(Boolean); // [1, 5] // is same as myArray.filter(x => x);
 

Answer:

You can get unique values of an array with the combination of Set and rest expression/spread(...) syntax.
 

console.log([...new Set([1, 2, 4, 4, 3])]); // [1, 2, 4, 3]
 

Answer:

Sometimes you would like to have a destructured variable with a different name than the property name. In that case, you'll use a : newName to specify a name for the variable. This process is called destructuring aliases.
 

const obj = { x: 1 };
// Grabs obj.x as as { otherName }
const { x: otherName } = obj;
 

Answer:

You can map the array values without using the map method by just using the from method of Array. Let's map city names from Countries array,
 

const countries = [
    { name: 'India', capital: 'Delhi' },
    { name: 'US', capital: 'Washington' },
    { name: 'Russia', capital: 'Moscow' },
    { name: 'Singapore', capital: 'Singapore' },
    { name: 'China', capital: 'Beijing' },
    { name: 'France', capital: 'Paris' },
];

const cityNames = Array.from(countries, ({ capital}) => capital);
console.log(cityNames); // ['Delhi, 'Washington', 'Moscow', 'Singapore', 'Beijing', 'Paris']
 

Answer:

You can empty an array quickly by setting the array length to zero.
 

let cities = ['Singapore', 'Delhi', 'London'];
cities.length = 0; // cities becomes []
 

Answer:

You can round numbers to a certain number of decimals using toFixed method from native javascript.
 

let pie = 3.141592653;
pie = pie.toFixed(3); // 3.142
 

Answer:

You can convert an array to an object with the same data using spread(...) operator.
 

var fruits = ["banana", "apple", "orange", "watermelon"];
var fruitsObject = {...fruits};
console.log(fruitsObject); // {0: "banana", 1: "apple", 2: "orange", 3: "watermelon"}
 

Answer:

You can create an array with some data or an array with the same values using fill method.
 

var newArray = new Array(5).fill("0");
console.log(newArray); // ["0", "0", "0", "0", "0"]
 

Answer:

Below are the list of placeholders available from console object,

  1. %o — It takes an object,
  2. %s — It takes a string,
  3. %d — It is used for a decimal or integer These placeholders can be represented in the console.log as below
const user = { "name":"John", "id": 1, "city": "Delhi"};
console.log("Hello %s, your details %o are available in the object form", "John", user); // Hello John, your details {name: "John", id: 1, city: "Delhi"} are available in object
 

Answer:

Yes, you can apply CSS styles to console messages similar to html text on the web page.
 

console.log('%c The text has blue color, with large font and red background', 'color: blue; font-size: x-large; background: red');
Note: All CSS styles can be applied to console messages.

Answer:

The console.dir() is used to display an interactive list of the properties of the specified JavaScript object as JSON.
 

const user = { "name":"John", "id": 1, "city": "Delhi"};
console.dir(user);
 

Answer:

Yes, it is possible to get and debug HTML elements in the console just like inspecting elements.
 

const element = document.getElementsByTagName("body")[0];
console.log(element);
 

Answer:

The console.table() is used to display data in the console in a tabular format to visualize complex arrays or objects.
 

const users = [{ "name":"John", "id": 1, "city": "Delhi"}, { "name":"Max", "id": 2, "city": "London"}, { "name":"Rod", "id": 3, "city": "Paris"} ];
console.table(users);

Not: Remember that console.table() is not supported in IE.

Answer:

The combination of IsNaN and isFinite methods are used to confirm whether an argument is a number or not.
 

function isNumber(n){
    return !isNaN(parseFloat(n)) && isFinite(n);
}
 

Answer:

You need to select the content(using .select() method) of the input element and execute the copy command with execCommand (i.e, execCommand('copy')). You can also execute other system commands like cut and paste.
 

document.querySelector("#copy-button").onclick = function() {
  // Select the content
  document.querySelector("#copy-input").select();
  // Copy to the clipboard
  document.execCommand('copy');
};
 

Answer:

You can use new Date().getTime() to get the current timestamp. There is an alternative shortcut to get the value.
 

console.log(+new Date());
console.log(Date.now());
 

Answer:

Flattening bi-dimensional arrays is trivial with Spread operator.
 

const biDimensionalArr = [11, [22, 33], [44, 55], [66, 77], 88, 99];
const flattenArr = [].concat(...biDimensionalArr); // [11, 22, 33, 44, 55, 66, 77, 88, 99]

But you can make it work with multi-dimensional arrays by recursive calls,
 
function flattenMultiArray(arr) {
    const flattened = [].concat(...arr);
    return flattened.some(item => Array.isArray(item)) ? flattenMultiArray(flattened) : flattened;
 }
const multiDimensionalArr = [11, [22, 33], [44, [55, 66, [77, [88]], 99]]];
const flatArr = flattenMultiArray(multiDimensionalArr); // [11, 22, 33, 44, 55, 66, 77, 88, 99]
 

Answer:

You can use indexOf to compare input with multiple values instead of checking each value as one condition.
 

// Verbose approach
if (input === 'first' || input === 1 || input === 'second' || input === 2) {
  someFunction();
}
// Shortcut
if (['first', 1, 'second', 2].indexOf(input) !== -1) {
  someFunction();
}
 

Answer:

The window.onbeforeunload method is used to capture browser back button events. This is helpful to warn users about losing the current data.
 

window.onbeforeunload = function() {
    alert("You work will be lost");
 };
 

Answer:

The right click on the page can be disabled by returning false from the oncontextmenu attribute on the body element.
 

<body oncontextmenu="return false;">
 

Answer:

Primitive Values like string,number and boolean don't have properties and methods but they are temporarily converted or coerced to an object(Wrapper object) when you try to perform actions on them. For example, if you apply toUpperCase() method on a primitive string value, it does not throw an error but returns uppercase of the string.
 

let name = "john";

console.log(name.toUpperCase());  // Behind the scenes treated as console.log(new String(name).toUpperCase());

i.e, Every primitive except null and undefined have Wrapper Objects and the list of wrapper objects are String,Number,Boolean,Symbol and BigInt.

Answer:

AJAX stands for Asynchronous JavaScript and XML and it is a group of related technologies(HTML, CSS, JavaScript, XMLHttpRequest API etc) used to display data asynchronously. i.e. We can send data to the server and get data from the server without reloading the web page.

Answer:

Below are the list of different ways to deal with Asynchronous code.

  1. Callbacks
  2. Promises
  3. Async/await
  4. Third-party libraries such as async.js,bluebird etc

Answer:

Until a few days back, One shortcoming of native promises is no direct way to cancel a fetch request. But the new AbortController from js specification allows you to use a signal to abort one or multiple fetch calls. The basic flow of cancelling a fetch request would be as below,

  1. Create an AbortController instance
  2. Get the signal property of an instance and pass the signal as a fetch option for signal
  3. Call the AbortController's abort property to cancel all fetches that use that signal For example, let's pass the same signal to multiple fetch calls will cancel all requests with that signal,
const controller = new AbortController();
const { signal } = controller;

fetch("http://localhost:8000", { signal }).then(response => {
    console.log(`Request 1 is complete!`);
}).catch(e => {
    if(e.name === "AbortError") {
        // We know it's been canceled!
    }
});

fetch("http://localhost:8000", { signal }).then(response => {
    console.log(`Request 2 is complete!`);
}).catch(e => {
     if(e.name === "AbortError") {
         // We know it's been canceled!
      }
});

// Wait 2 seconds to abort both requests
setTimeout(() => controller.abort(), 2000);
 

Answer:

Web speech API is used to enable modern browsers recognize and synthesize speech(i.e, voice data into web apps). This API has been introduced by W3C Community in the year 2012. It has two main parts,
1. SpeechRecognition (Asynchronous Speech Recognition or Speech-to-Text): It provides the ability to recognize voice context from an audio input and respond accordingly. This is accessed by the SpeechRecognition interface. The below example shows on how to use this API to get text from speech,
 

window.SpeechRecognition = window.webkitSpeechRecognition || window.SpeechRecognition;  // webkitSpeechRecognition for Chrome and SpeechRecognition for FF
const recognition = new window.SpeechRecognition();
recognition.onresult = (event) => { // SpeechRecognitionEvent type
  const speechToText = event.results[0][0].transcript;
  console.log(speechToText);
}
recognition.start();

In this API, browser is going to ask you for permission to use your microphone

1. SpeechSynthesis (Text-to-Speech): It provides the ability to recognize voice context from an audio input and respond. This is accessed by the SpeechSynthesis interface. For example, the below code is used to get voice/speech from text,
 
if('speechSynthesis' in window){
    var speech = new SpeechSynthesisUtterance('Hello World!');
    speech.lang = 'en-US';
    window.speechSynthesis.speak(speech);
}

The above examples can be tested on chrome(33+) browser's developer console. Note: This API is still a working draft and only available in Chrome and Firefox browsers(ofcourse Chrome only implemented the specification)

Answer:

Both browser and NodeJS javascript environments throttles with a minimum delay that is greater than 0ms. That means even though setting a delay of 0ms will not happen instantaneously. Browsers: They have a minimum delay of 4ms. This throttle occurs when successive calls are triggered due to callback nesting(certain depth) or after a certain number of successive intervals. Note: The older browsers have a minimum delay of 10ms. Nodejs: They have a minimum delay of 1ms. This throttle happens when the delay is larger than 2147483647 or less than 1. The best example to explain this timeout throttling behavior is the order of below code snippet.
 

function runMeFirst() {
    console.log('My script is initialized');
}
setTimeout(runMeFirst, 0);
console.log('Script loaded');

and the output would be in
 
Script loaded
My script is initialized
If you don't use setTimeout, the order of logs will be sequential.
 
function runMeFirst() {
   console.log('My script is initialized');
}
runMeFirst();
console.log('Script loaded');

and the output is,
 
My script is initialized
Script loaded

Answer:

You can't use setTimeout(fn, 0) to execute the code immediately due to minimum delay of greater than 0ms. But you can use window.postMessage() to achieve this behavior.

Answer:

A task is any javascript code/program which is scheduled to be run by the standard mechanisms such as initially starting to run a program, run an event callback, or an interval or timeout being fired. All these tasks are scheduled on a task queue. Below are the list of use cases to add tasks to the task queue,

  1. When a new javascript program is executed directly from console or running by the <script> element, the task will be added to the task queue.
  2. When an event fires, the event callback added to task queue
  3. When a setTimeout or setInterval is reached, the corresponding callback added to task queue

Answer:

Microtask is the javascript code which needs to be executed immediately after the currently executing task/microtask is completed. They are kind of blocking in nature. i.e, The main thread will be blocked until the microtask queue is empty. The main sources of microtasks are Promise.resolve, Promise.reject, MutationObservers, IntersectionObservers etc

Note: All of these microtasks are processed in the same turn of the event loop

Answer:

It is known that not all JavaScript libraries or frameworks have TypeScript declaration files. But if you still want to use libraries or frameworks in our TypeScript files without getting compilation errors, the only solution is declare keyword along with a variable declaration. For example, let's imagine you have a library called customLibrary that doesn’t have a TypeScript declaration and have a namespace called customLibrary in the global namespace. You can use this library in typescript code as below,
 

declare var customLibrary;

In the runtime, typescript will provide the type to the customLibrary variable as any type. The another alternative without using declare keyword is below
 
var customLibrary: any;
 

Answer:

Some of the major difference in a tabular form
 

Promises Observables
Emits only a single value at a time Emits multiple values over a period of time(stream of values ranging from 0 to multiple)
Eager in nature; they are going to be called immediately Lazy in nature; they require subscription to be invoked
Promise is always asynchronous even though it resolved immediately Observable can be either synchronous or asynchronous
Doesn't provide any operators Provides operators such as map, forEach, filter, reduce, retry, and retryWhen etc
Cannot be canceled Canceled by using unsubscribe() method

Answer:

Heap(Or memory heap) is the memory location where objects are stored when we define variables. i.e, This is the place where all the memory allocations and de-allocation take place. Both heap and call-stack are two containers of JS runtime. Whenever runtime comes across variables and function declarations in the code it stores them in the Heap.

Answer:

Event Table is a data structure that stores and keeps track of all the events which will be executed asynchronously like after some time interval or after the resolution of some API requests. i.e Whenever you call a setTimeout function or invoke async operation, it is added to the Event Table. It doesn't not execute functions on it’s own. The main purpose of the event table is to keep track of events and send them to the Event Queue as shown in the below diagram.

Answer:

Microtask Queue is the new queue where all the tasks initiated by promise objects get processed before the callback queue. The microtasks queue are processed before the next rendering and painting jobs. But if these microtasks are running for a long time then it leads to visual degradation.

Answer:

A shim is a library that brings a new API to an older environment, using only the means of that environment. It isn't necessarily restricted to a web application. For example, es5-shim.js is used to emulate ES5 features on older browsers (mainly pre IE9). Whereas polyfill is a piece of code (or plugin) that provides the technology that you, the developer, expect the browser to provide natively. In a simple sentence, A polyfill is a shim for a browser API.

Answer:

In JavaScript, primitive types include boolean, string, number, BigInt, null, Symbol and undefined. Whereas non-primitive types include the Objects. But you can easily identify them with the below function,
 

var myPrimitive = 30;
var myNonPrimitive = {};
function isPrimitive(val) {
    return Object(val) !== val;
}

isPrimitive(myPrimitive);
isPrimitive(myNonPrimitive);

If the value is a primitive data type, the Object constructor creates a new wrapper object for the value. But If the value is a non-primitive data type (an object), the Object constructor will give the same object.

Answer:

Babel is a JavaScript transpiler to convert ECMAScript 2015+ code into a backwards compatible version of JavaScript in current and older browsers or environments. Some of the main features are listed below,

  1. Transform syntax
  2. Polyfill features that are missing in your target environment (using @babel/polyfill)
  3. Source code transformations (or codemods)

Answer:

Node is a single thread, but some of the functions included in the Node.js standard library(e.g, fs module functions) are not single threaded. i.e, Their logic runs outside of the Node.js single thread to improve the speed and performance of a program.

Answer:

Some of the most common use cases of observables are web sockets with push notifications, user input changes, repeating intervals, etc

Answer:

RxJS (Reactive Extensions for JavaScript) is a library for implementing reactive programming using observables that makes it easier to compose asynchronous or callback-based code. It also provides utility functions for creating and working with observables.

Answer:

The functions which are created with Function constructor do not create closures to their creation contexts but they are always created in the global scope. i.e, the function can access its own local variables and global scope variables only. Whereas function declarations can access outer function variables(closures) too.

Let's see this difference with an example,
Function Constructor:

 

var a = 100;
function createFunction() {
    var a = 200;
    return new Function('return a;');
}
console.log(createFunction()()); // 100

Function declaration:
 
var a = 100;
function createFunction() {
    var a = 200;
    return function func() {
        return a;
    }
}
console.log(createFunction()()); // 200
 

Answer:

Short circuit conditions are meant for condensed way of writing simple if statements. Let's demonstrate the scenario using an example. If you would like to login to a portal with an authentication condition, the expression would be as below,
 

if (authenticate) {
   loginToPorta();
}

Since the javascript logical operators evaluated from left to right, the above expression can be simplified using && logical operator
 
authenticate && loginToPorta();
 

Answer:

The length property of an array is useful to resize or empty an array quickly. Let's apply length property on number array to resize the number of elements from 5 to 2,
 

var array = [1, 2, 3, 4, 5];
console.log(array.length); // 5

array.length = 2;
console.log(array.length); // 2
console.log(array); // [1,2]

and the array can be emptied too
 
var array = [1, 2, 3, 4, 5];
array.length = 0;
console.log(array.length); // 0
console.log(array); // []
 

Answer:

An Observable is basically a function that can return a stream of values either synchronously or asynchronously to an observer over time. The consumer can get the value by calling subscribe() method. Let's look at a simple example of an Observable
 

import { Observable } from 'rxjs';

const observable = new Observable(observer => {
  setTimeout(() => {
    observer.next('Message from a Observable!');
  }, 3000);
});

observable.subscribe(value => console.log(value));
 

Answer:

The main difference between function declarations and class declarations is hoisting. The function declarations are hoisted but not class declarations.
Classes:
 

const user = new User(); // ReferenceError

class User {}

Constructor Function:
 
const user = new User(); // No error

 function User() {
 }
 

Answer:

An async function is a function declared with the async keyword which enables asynchronous, promise-based behavior to be written in a cleaner style by avoiding promise chains. These functions can contain zero or more await expressions.

Let's take a below async function example,

 

async function logger() {

  let data = await fetch('http://someapi.com/users'); // pause until fetch returns
  console.log(data)
}
logger();

It is basically syntax sugar over ES2015 promises and generators.

Answer:

While using asynchronous code, JavaScript’s ES6 promises can make your life a lot easier without having callback pyramids and error handling on every second line. But Promises have some pitfalls and the biggest one is swallowing errors by default.

Let's say you expect to print an error to the console for all the below cases,

 

Promise.resolve('promised value').then(function() {
      throw new Error('error');
});

Promise.reject('error value').catch(function() {
      throw new Error('error');
});

new Promise(function(resolve, reject) {
      throw new Error('error');
});

But there are many modern JavaScript environments that won't print any errors. You can fix this problem in different ways,
1. Add catch block at the end of each chain: You can add catch block to the end of each of your promise chains
 
Promise.resolve('promised value').then(function() {
    throw new Error('error');
}).catch(function(error) {
  console.error(error.stack);
});
But it is quite difficult to type for each promise chain and verbose too.

2. Add done method: You can replace first solution's then and catch blocks with done method
 
Promise.resolve('promised value').done(function() {
    throw new Error('error');
});

Let's say you want to fetch data using HTTP and later perform processing on the resulting data asynchronously. You can write done block as below,
 
getDataFromHttp()
  .then(function(result) {
    return processDataAsync(result);
  })
  .done(function(processed) {
    displayData(processed);
  });

In future, if the processing library API changed to synchronous then you can remove done block as below,
 
getDataFromHttp()
   .then(function(result) {
     return displayData(processDataAsync(result));
   })
and then you forgot to add done block to then block leads to silent errors.

3. Extend ES6 Promises by Bluebird: Bluebird extends the ES6 Promises API to avoid the issue in the second solution. This library has a “default” onRejection handler which will print all errors from rejected Promises to stderr. After installation, you can process unhandled rejections
 
Promise.onPossiblyUnhandledRejection(function(error){
    throw error;
});

and discard a rejection, just handle it with an empty catch
 
Promise.reject('error value').catch(function() {});
 

Answer:

Deno is a simple, modern and secure runtime for JavaScript and TypeScript that uses V8 JavaScript engine and the Rust programming language.

Answer:

By default, plain objects are not iterable. But you can make the object iterable by defining a Symbol.iterator property on it.

Let's demonstrate this with an example,

 

onst collection = {
  one: 1,
  two: 2,
  three: 3,
  [Symbol.iterator]() {
    const values = Object.keys(this);
    let i = 0;
    return {
      next: () => {
        return {
          value: this[values[i++]],
          done: i > values.length
        }
      }
    };
  }
};

const iterator = collection[Symbol.iterator]();

console.log(iterator.next());    // ? {value: 1, done: false}
console.log(iterator.next());    // ? {value: 2, done: false}
console.log(iterator.next());    // ? {value: 3, done: false}
console.log(iterator.next());    // ? {value: undefined, done: true}

The above process can be simplified using a generator function,
 
const collection = {
   one: 1,
   two: 2,
   three: 3,
   [Symbol.iterator]: function * () {
     for (let key in this) {
       yield this[key];
     }
   }
 };
 const iterator = collection[Symbol.iterator]();
 console.log(iterator.next());    // {value: 1, done: false}
 console.log(iterator.next());    // {value: 2, done: false}
 console.log(iterator.next());    // {value: 3, done: false}
 console.log(iterator.next());    // {value: undefined, done: true}
 

Answer:

First, we should know about tail call before talking about "Proper Tail Call". A tail call is a subroutine or function call performed as the final action of a calling function. Whereas Proper tail call(PTC) is a technique where the program or code will not create additional stack frames for a recursion when the function call is a tail call.

For example, the below classic or head recursion of factorial function relies on stack for each step. Each step need to be processed upto n * factorial(n - 1)

 

function factorial(n) {
  if (n === 0) {
    return 1
  }
  return n * factorial(n - 1)
}
console.log(factorial(5)); //120

But if you use Tail recursion functions, they keep passing all the necessary data it needs down the recursion without relying on the stack.
 
function factorial(n, acc = 1) {
  if (n === 0) {
    return acc
  }
  return factorial(n - 1, n * acc)
}
console.log(factorial(5)); //120

The above pattern returns the same output as the first one. But the accumulator keeps track of total as an argument without using stack memory on recursive calls.

Answer:

If you don't know if a value is a promise or not, wrapping the value as Promise.resolve(value) which returns a promise

 

function isPromise(object){
      if(Promise && Promise.resolve){
      return Promise.resolve(object) == object;
      }else{
      throw "Promise not supported in your environment"
      }
   }

   var i = 1;
   var promise = new Promise(function(resolve,reject){
      resolve()
   });

   console.log(isPromise(i)); // false
   console.log(isPromise(p)); // true

Another way is to check for .then() handler type
 
function isPromise(value) {
   return Boolean(value && typeof value.then === 'function');
}
var i = 1;
var promise = new Promise(function(resolve,reject){
   resolve()
});

console.log(isPromise(i)) // false
console.log(isPromise(promise)); // true
 

Answer:

You can use new.target pseudo-property to detect whether a function was called as a constructor(using the new operator) or as a regular function call.
1. If a constructor or function invoked using the new operator, new.target returns a reference to the constructor or function.
2. For function calls, new.target is undefined.

function Myfunc() {
   if (new.target) {
      console.log('called with new');
   } else {
      console.log('not called with new');
   }
}

new Myfunc(); // called with new
Myfunc(); // not called with new
Myfunc.call({}); not called with new
 

Answer:

There are three main differences between arguments object and rest parameters

  1. The arguments object is an array-like but not an array. Whereas the rest parameters are array instances.
  2. The arguments object does not support methods such as sort, map, forEach, or pop. Whereas these methods can be used in rest parameters.
  3. The rest parameters are only the ones that haven’t been given a separate name, while the arguments object contains all arguments passed to the function

Answer:

Rest parameter collects all remaining elements into an array. Whereas Spread operator allows iterables( arrays / objects / strings ) to be expanded into single arguments/elements. i.e, Rest parameter is opposite to the spread operator.

Answer:

There are five kinds of generators,
1. Generator function declaration:
 

function* myGenFunc() {
      yield 1;
      yield 2;
      yield 3;
 }
 const genObj = myGenFunc();

2. Generator function expressions:
const myGenFunc = function* () {
       yield 1;
       yield 2;
       yield 3;
};
const genObj = myGenFunc();

3. Generator method definitions in object literals:
 
const myObj = {
     * myGeneratorMethod() {
        yield 1;
        yield 2;
        yield 3;
     }
 };
 const genObj = myObj.myGeneratorMethod();

4. Generator method definitions in class:
 
class MyClass {
     * myGeneratorMethod() {
        yield 1;
        yield 2;
        yield 3;
     }
  }
  const myObject = new MyClass();
  const genObj = myObject.myGeneratorMethod();

5. Generator as a computed property:
 
const SomeObj = {
  *[Symbol.iterator] () {
    yield 1;
    yield 2;
    yield 3;
  }
}

console.log(Array.from(SomeObj)); // [ 1, 2, 3 ]
 

Answer:

Below are the list of built-in iterables in javascript,

  1. Arrays and TypedArrays
  2. Strings: Iterate over each character or Unicode code-points
  3. Maps: iterate over its key-value pairs
  4. Sets: iterates over their elements
  5. arguments: An array-like special variable in functions
  6. DOM collection such as NodeList

Answer:

Both for...in and for...of statements iterate over js data structures. The only difference is over what they iterate:

  1. for..in iterates over all enumerable property keys of an object
  2. for..of iterates over the values of an iterable object.

Let's explain this difference with an example,

 

let arr = ['a', 'b', 'c'];

arr.newProp = 'newVlue';

// key are the property keys
for (let key in arr) {
  console.log(key);
}

// value are the property values
for (let value of arr) {
  console.log(value);
}

Since for..in loop iterates over the keys of the object, the first loop logs 0, 1, 2 and newProp while iterating over the array object. The for..of loop iterates over the values of a arr data structure and logs a, b, c in the console.

Answer:

The Instance properties must be defined inside of class methods. For example, name and age properties defined insider constructor as below,
 

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
}

But Static(class) and prototype data properties must be defined outside of the ClassBody declaration. Let's assign the age value for Person class as below,
 
Person.staticAge = 30;
Person.prototype.prototypeAge = 40;
 

Answer:

  1. isNaN: The global function isNaN converts the argument to a Number and returns true if the resulting value is NaN.
  2. Number.isNaN: This method does not convert the argument. But it returns true when the type is a Number and value is NaN.
isNaN(‘hello’);   // true
Number.isNaN('hello'); // false
 

Answer:

Immediately Invoked Function Expressions(IIFE) requires a pair of parenthesis to wrap the function which contains set of statements.
 

(function(dt) { 
   console.log(dt.toLocaleTimeString()); 
 })(new Date()); 

Since both IIFE and void operator discard the result of an expression, you can avoid the extra brackets using void operator for IIFE as below,   
 
void function(dt) { 
   console.log(dt.toLocaleTimeString()); 
 }(new Date()); 
 

Answer:

You might have seen expressions used in switch condition but it is also possible to use for switch cases by assigning true value for the switch condition. Let's see the weather condition based on temparature as an example,
 

onst weather = function getWeather(temp) {
   switch(true) {
       case temp < 0: return 'freezing';
       case temp < 10: return 'cold';
       case temp < 24: return 'cool';
       default: return 'unknown';
   }
   }(10);
 

Answer:

The easiest and safest way to ignore promise errors is void that error. This approach is ESLint friendly too.
 

await promise.catch(e => void e);
 

Answer:

You can add CSS styling to the console output using the CSS format content specifier %c. The console string message can be appended after the specifier and CSS style in another argument. Let's print the red the color text using console.log and CSS specifier as below,
 

console.log("%cThis is a red text", "color:red");

It is also possible to add more styles for the content. For example, the font-size can be modified for the above text
 
console.log("%cThis is a red text with bigger font", "color:red; font-size:20px");