Common JavaScript Interview Questions and Answers [PDF]

Hey, are you a javascript developer or preparing for your interviews so here I am going to share the "Top 15 Common Asked JavaScript Interview Questions with Answers". And also you can download the PDF of all these Interview Questions with Answers.

So for what you are waiting "Unlock Your Potential as a JavaScript Developer: Ace Your Next Interview with These Essential Questions and Answers". These questions are collected from different websites and articles and in this I shared the most commonly asked questions with answers.

1. What is JavaScript and what is it used for?

JavaScript is a high-level, programming language. It is used to create dynamic, interactive web pages. It is often run on the client side in a web browser and now it can also be run server-side via Node.js.

2. What is a closure in JavaScript?

A closure is a function that has access to variables in its outer scope, even after the outer function has returned. In other words, a closure gives you access to an outer function's scope from an inner function.

Closure example using JavaScript:

function outerFunction(a) {

    return function innerFunction(b) {

        return a + b;

    }

}

const add20 = outerFunction(20);

console.log(add20(5));

console.log(add20(20))

3. Can you explain the difference between let and var in JavaScript?

'var' is function scoped, while let and const are block scoped. var is also hoisted, while let and const are not.

4. What is the difference between == and === in JavaScript?

'==' performs type conversion when comparing two things, which means it converts the values of different types to a common type before comparison.

'===' performs strict equality comparison, meaning it only returns true if the operands have the same value and type.

JavaScript example using '==' and '==='

console.log(4 == "4");

console.log(4 === "4");

5. Can you explain the event loop in JavaScript?

In JavaScript event loop is a mechanism that handles the execution of code and determines the order in which code is executed. It is responsible for executing the code, collecting and processing events, and executing queued sub-tasks.

Event loop example javascript:

console.log('Starting from here...');

setTimeout(function () {

    console.log('The setTimeout callback');

}, 4000);

console.log('Ending here...');

6. Can you explain the difference between null and undefined in JavaScript?

In javascript 'null' is an object and it is explicitly used to represent the absence of value.

'undefined' means a variable has been declared but has not been assigned a value.

'null' and 'undefined' examples:

let a;

console.log(a);

let b = null;

console.log(b);

7. What is a higher-order function in JavaScript?

In javascript, a function which takes another function as an argument or returns a function is known as a higher-order function. Higher-order functions allow for more abstract and flexible code.

Javascript higher-order function example:

function multiplyBy(factor) {

    return function (num) {

        return num * factor;

    }

}

const double = multiplyBy(2);

console.log(double(5));

const triple = multiplyBy(3);

console.log(triple(5));

8. What is the difference between synchronous and asynchronous code in JavaScript?

Synchronous code is executed in a given order, meaning each statement is executed one after the other. And Asynchronous code allows multiple events to occur simultaneously and out of order, without blocking the execution of other code.

Synchronous and Asynchronous example in javascript:

// Synchronous code of JavaScript

console.log('Start');

for (let i = 0; i < 20; i++) {

    console.log(i)

}

console.log('End');

// Asynchronous code of JavaScript

console.log('Start');

setTimeout(function () {

    console.log('Timeout callback');

}, 1000);

console.log('End');

9. How to declare a variable in JavaScript?

In JavaScript, a variable can be declared using the var, let, or const keywords. 

The var keyword is used to declare a variable that is accessible within the entire function in which it is declared, or globally if declared outside of a function.

Declaring variable using var:

var a = 5;

console.log(a);

In modern JavaScript (ES6), we can also declare variables using the let or const keywords instead of var.

Declaring variable using let:

let b = 10;

console.log(b);

Declaring variable using const:

const c = 15;

console.log(c);

10. What is hoisting in JavaScript?

Hoisting is a behavior in JavaScript where variable and function declarations are moved to the top of their scope.

In JavaScript, it is the process whereby the interpreter appears to move the declaration of functions, variables, or classes to the top of their scope, prior to the execution of the code.

11. What is a callback function in JavaScript?

A callback function is a function that is passed as an argument to another function. It is executed after the outer function has finished executing. In JavaScript callback function allows for the creation of asynchronous and non-blocking code.

callback in javascript example:

// function

function greeting(name, callback) {

    console.log('Hi' + ' ' + name);

    callback();

}

// callback function

function callMe() {

    console.log('I am a callback function');

}

// passing function as an argument

greeting('Raju Webdev', callMe);

12. What is the use of this keyword in JavaScript?

The 'this' keyword in javascript refers to an object that is executing the current function. The 'this' keyword refers to different objects depending on how it is used. And Its value depends on the context in which it is used.

this keyword example in javascript:

const myObj = {

    name: "Raju",

    callName: function () {

        console.log(this.name);

    }

};

myObj.callName();

13. What is the difference between call and apply methods in JavaScript?

In javascript, 'call' and 'apply' are used to invoke a function and set the 'this' value. The main difference between call and apply is that, 'call' takes arguments separately, while 'apply' takes them as an array.

14. What is the difference between ViewState and SessionState?

• 'ViewState' is specific to a page in a session. And it is maintained at the page level only.

• 'SessionState' is specific to user-specific data that can be accessed across all web application pages. And it is maintained at the session level. 

15. How to read and write a file using JavaScript?

In javascript there are two ways to read and write a file:

• Using JavaScript extensions which runs from the javascript editor

• Using a web page and Active X objects

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