The Ultimate JavaScript Interview Questions Guide: Preparing for Success

Looking to ace your next JavaScript interview? Our Ultimate JavaScript Interview Questions Guide has got you covered! Our comprehensive guide covers everything from the basics of JavaScript to more advanced topics like closures and prototypes. We’ve compiled a list of the most frequently asked JavaScript interview questions and provided detailed explanations and examples to help you prepare for success. Whether you’re a beginner or an experienced developer, our guide will give you the confidence and knowledge you need to land your dream job. Don’t miss out on this valuable resource – start preparing for your JavaScript interview today!

Top JavaScript Interview Questions for Beginners

  1. What is a Java Script?

JavaScript is a programming language that enables users to add complex features to web pages. The user bet that JavaScript is involved whenever a web page does more functions than show you static information; it can indicate you have updated content, interactive maps, animated 2D/3D graphics, scrolling video jukeboxes, etc. It is the third layer of standard web technologies. It enables you to design dynamically and update content with control over multimedia and animated images.

2. What is the procedure for declaring a variable in Javascript?

A variable can be declared using the ‘var,’ ‘let,’ or ‘const’ keywords, then a variable name. e.g.:

var myVar;

let meet;

const const;

3. Differentiate between var and let?

The most crucial difference between let and var is that variables declared with let

can only be used in the block of code where they were announced, while variables

declared with var can be used in the whole function where they were displayed.

4. What do you mean by the array in Javascript?

An array is a structure of data that holds more than one value of any type in a single variable; these values are kept in a list, and each value is known by index number.

5. How to add an element to an array?

Users can add an element to an array using the ‘push()’ method, which adds an element at the end of an array, e.g.:

let myArray = [1, 2, 3];


// myArray is now [1, 2, 3, 4]

6. How to define a function in javascript?

The function is the block of code that performs a defined task. It takes inputs, also known as parameters, and then makes output. Code is enclosed in curly braces {}. E.g.:

function myFunction(param1, param2) {

// code to be executed


7. What to do in JavaScript to call a function?

In JavaScript, the name of a function is followed by parenthesis () that can hold arguments (if the function has parameters). Examples:

myFunction(argument1, argument2);

8. Tell me about the object in JavaScript.

In JavaScript, objects are collections of key-value pairs, where the keys are strings or symbols, and the values can be any data. Objects are used to keep information and related features in one place.

9. How to access an object property in javascript?

There is a simple way to access an object property in javascript using dot notation or bracket notation. E.g.:

let myObject = {name: “John,” age: 30};

console.log(; // prints “John”

console.log(myObject[“age”]); // prints 30

10. Why the word debugger is used in javascript?

The browser’s debugger must be turned on to debug the code. Some debuggers can be turned on if a problem is reported. During debugging, the rest of the code must be stopped before going to the following line.

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Advanced JavaScript Interview Questions

  1. What is equality in javascript?

In JavaScript, you can make both strict and type-changing comparisons:

Value equivalence can be checked with a tough comparison (===), which makes it impossible to cheat.

Abstract comparison operators (==) can determine if two values are equal and coercion is allowed.

Guidelines for fundamental equality:

Do not use == if either of the compared values could be right. Instead, use ===.

If either value could be zero, an empty string, or an empty array, use === instead of ==.

If you need more clarification, just use ==. It’s risk-free, but it also makes your code more straightforward, making it easier to understand in many situations.

  1. What do you mean by event bubbling, and why is it used in javascript?

Simply put, event bubbling is the way events move up the DOM tree from the lowest level to the highest level. When an event happens on the element, then it runs any attached event handlers for that element, then moves up the hierarchy to the parent element and runs any attached event handlers for that element, and so on, until it reaches the document object or a handler calls the stopPropagation() method.

  1. Does JavaScript support tasks that run at different times? Could you say more about the event loop?

JavaScript uses a single-threaded event loop for tasks that don’t happen simultaneously. In the event loop, each event that comes in is checked for until it is processed. Once an asynchronous task starts, each new one is added to a queue. As soon as the event loop handles the operation, the corresponding callback function will be run. This means that other processes can run in the background without being stopped while the asynchronous operation is being done.

  1. Please explain the difference between prototypal and classical inheritance in JavaScript.

In classical inheritance, classes are made first, and then objects are created from those classes. On the other hand, prototypical inheritance is based on making objects and then taking on their traits and behaviors. Inheritance in JavaScript is more flexible and dynamic because objects can inherit directly from each other.

  1. Explain closure in Javascript and how does it work?

In Javascript, a closure is a function that keeps access to the variables in its outer (enclosing) function even after the external position has returned. When a function returns another part, the internal process keeps track of the variables called it. This is called a “closure.” By doing this, the inner part can use the variables even after leaving the scope of the function called it.

  1. What is the output of the code below? 

  //mph (named function expression)

var Foo = Function Bar()


return 7;


typeof Bar();

 Reference Error is the expected result. Only one reference variable can be used as the function name in a given function definition.

  1. Why does a function book have to include the entire contents of a JavaScript source file?

It’s a standard method that a lot of JavaScript modules use. The most important thing about this method is that it creates a private namespace for the file’s contents. This helps keep different JavaScript libraries and modules from having too similar names.

This method can also be used as an alias for a global variable, which is very useful. Because of this, it is often used in jQuery add-ons.

8. What do you mean by escape characters in javascript?

Using “escape characters” makes it safe to use special characters in your JavaScript code. Escape characters are used to interact with non-printing characters, such as single quotes, double quotes, apostrophes, and ampersands (Backslash). Type a backslash before the characters to see them. For example-

Document. write, “I am a boy.”

document. Write, “I am a boy.”

9. What are self-invoking functions in javascript?

A self-invoking expression runs independently without the user having to do anything (initiated). Putting () after a function expression makes the function run right away. A function declaration can’t be used as an actual call.

Even though functions are usually declared and called, anonymous functions can run a position only once when specified without needing a separate call. Also, these kinds of operations don’t have names.

10. What are strict modes in javascript, and are the characteristics the same?

Strict Mode is a new part of JavaScript that lets you write code or functions that only run in a particular set of circumstances. Most of the time, this phrase is used to throw less severe mistakes. But in “Strict mode,” any problem will be reported, even ones that don’t show up. Because of this, it is much easier to find bugs and fix them. This means that a coder is less likely to make a mistake.

critical points about JavaScript’s strict Mode

  • The developers will only let you use the same argument once.
  • In the hard way, JavaScript cannot be used as a parameter or function name.
  • The keyword “use strict” defines strict Mode for the first time. Strict Mode can be used with every browser out there.
  • In “Strict Mode,” engineers can’t set any global variables.

Tricky JavaScript Interview Questions

  1. Differentiate between the == and === operators in JavaScript?

The correct answer is that == is a fuzzy comparison operator that works with values of different types. The === equality operator compares two values one to one and checks for type safety to see if they are the same. For example, the statement 1 == “1” is correct, but the view 1 === “1” is wrong.

2.What do you mean by event delegation in JavaScript?

Using the JavaScript event delegation method, you can avoid attaching event handlers to each child element individually. Instead, you can delegate events to a parent element instead of doing so. Because of this, the amount of code required for event processing is reduced, making the process more straightforward to implement. Event delegation can function because it uses event propagation, a process in which events are first handled by the child element that triggered the event before being transmitted up the Document Object Model (DOM) tree to the parent element. This process is called “event propagation.”

3. What do you mean by NaN property?

It means not a number value. Indicates that it is not a legal number

The type of this NaN will return the Number.

To see if a value is NaN, we use the isNaN() function,

Note- isNaN() function converts the given value to a Number type, then equates to NaN.

isNaN(“Hello”) // Returns true

isNaN(345) // Returns false

isNaN(‘1’) // Returns false since ‘1’ is converted to Number type, which results in 0 ( a number)

isNaN(true) // Returns false since true converted to Number type results in 1 ( a number)

isNaN(false) // Returns false

isNaN(undefined) // Returns true

4. What is passed by reference and passed by value?

To understand the difference between passing data by value and passing data by reference, you need to know how to make a variable and assign it a value.

var x = 2;

On the line of code that just came before this one, we defined an x variable and assigned the value 2 to it. In the background, the “=” symbol, also known as the assign operator, creates a new memory location, then stores the value “2” before returning the address of the newly created memory. Because of this, the x character in the line of code that came before it indicates the address of the memory location rather than referring to the number 2.

5. What do you mean by immediately invoked function?

An invoked function is a function that runs after it is defined.

Syntax of IIFE :


// Do something;


To understand IIFE, we need to understand the two sets of parentheses that are added while creating an IIFE :

The first set of parenthesis:

(function (){

//Do something;


While executing javascript code, whenever the Compiler sees the word “function,” it assumes that we are declaring a function in the code. Therefore, if we do not use the first set of parentheses, the Compiler throws an error because it thinks we are declaring a function, and by the syntax of declaring a part, a process should always have a name.

function() {

//Do something;


//Compiler gives an error since the syntax of declaring a function is wrong in the code above.

To remove this error, we add the first set of parenthesis that tells the Compiler that the function is not a function declaration. Instead, it’s a function expression.

The second set of parenthesis:

(function (){

//Do something;


From the definition of an IIFE, we know that our code should run as soon as it is defined. A function runs only when it is invoked. If we do not gather the process, the function declaration is returned:

(function (){

// Do something;


// Returns the function declaration

Therefore to invoke the function, we use the second set of parenthesis.


 The importance of javascript in today’s world is increasing daily, and understanding javascript for programmers and developers is fundamental. They should go through this post on Top javascript interview questions to perform well in javascript interviews.

Everyone who wants to work as a web developer needs to know these most-asked JavaScript interview questions. Some of the drawbacks of these questions are that they are object-oriented programming, closures, and asynchronous code.

Learning through these questions is a great way to gain knowledge of the language, the ability to solve problems, and the ability to think critically. This will set you apart from other web developers and increase your hiring chance.

Also, practicing answers to common JavaScript interview questions can help you see where you need to improve. Even if you don’t get the job, this can help you to learn the language faster.

To make it as a web developer, you should spend enough time learning how to answer the most common JavaScript interview questions. Candidates have a better chance of getting hired and moving up in this exciting field if they can show that you know the language well.

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