Top JavaScript Interview Questions and Answers


Top JavaScript Interview Questions and Answers

JavaScript is one of the world’s most popular programming languages, powering many websites and apps on the internet. As a result, it has become a standard element of the interview process for many software development positions, particularly those involving front-end web development. 

Whether you’re an experienced JavaScript developer or just starting, it’s critical to be prepared for the kind of javascript technical interview questions you could face in a job interview. In this post, we’ll look at some of the most common JavaScript interview questions and give extensive responses to help you feel confident and prepared. 

Whether you’re interviewing for your first or tenth job, this book will help you approach the interview with a firm knowledge of certain fundamental JavaScript ideas and principles.

Basic Level Interview Questions

Here are some frequently asked questions in the interview.

  • What is JavaScript, and how does it differ from Java?

JavaScript is a scripting language used for web development, while Java is a general-purpose programming language used for building standalone applications. JavaScript is a client-side language, while Java can be used both on the client and server side.

  • What are the basic data types in JavaScript?

The basic data types in JavaScript are string, number, boolean, null, undefined, symbol (introduced in ES6), and object.

  • What is the difference between null and undefined in JavaScript?

In JavaScript, null is a value that represents the intentional absence of any object value, while undefined is a value assigned to a variable that has not been assigned any other value.

  • How do you declare a variable in JavaScript?

In JavaScript, you can declare variables by using the var, let, or const keywords followed by the variable name.

  • What is the difference between var, let, and const in JavaScript?

var is function-scoped and can be re-declared and updated, while let and const are block-scoped and cannot be re-declared but can be updated (in the case of let) or cannot be updated (in the case of const).

  • What is hoisting in JavaScript?

Hoisting is a mechanism in JavaScript where variable and function declarations are moved to the top of their respective scopes before code execution.

  • What is the difference between == and === in JavaScript?

== is a loose equality comparison operator that checks if two values are equal, while === is a strict equality comparison operator that checks if two values are equal in value and data type.

  • What is an array in JavaScript?

An array in JavaScript is a data structure that can store a collection of values of different data types.

  • How do you access elements in an array in JavaScript?

You can access elements in an array in JavaScript using square brackets and the index of the element you want to access, starting from 0.

  • What is a function in JavaScript?

A function in JavaScript is a block of code that can be called and executed to perform a specific task.

  • How do you define a function in JavaScript?

You can define a function in JavaScript using the function keyword followed by the function name, parameters (if any), and the code block enclosed in curly braces.

  • What is a callback function in JavaScript?

A callback function in JavaScript is a function passed as an argument to another function, which is then executed by the receiving function.

  • What is an object in JavaScript?

An object in JavaScript is a collection of key-value pairs, where each key is a string (or symbol) and each value can be of any data type.

  • In JavaScript, how can you get at an object’s properties?

In JavaScript, you may access an object’s properties using dot notation ( or bracket notation (object[‘property’]).

  • In JavaScript, what is the distinction between a method and a function?

A method is a property of an object that is a function, whereas a function can be a standalone or a property of an object.

  • What is the DOM in JavaScript?

The JavaScript Document Object Model (DOM) is an object-oriented representation of HTML components in a web page that can be changed using JavaScript.

  • What is an event in JavaScript?

An event in JavaScript is a user action or system occurrence that causes the browser or web page to respond.

  • In JavaScript, what does AJAX mean?

By sending asynchronous queries to the server, a web page may be updated without requiring a complete page reload thanks to the Asynchronous JavaScript and XML (AJAX) method in JavaScript.

  • What does JavaScript JSON mean?

JavaScript’s JSON (JavaScript Object Notation) is a lightweight data transfer format that is simple for both people and machines to read, write, parse, and produce.

  • What are the different types of operators in JavaScript, and how can they be used to manipulate values?

Several kinds of operators are available in JavaScript that can be used to change values. For instance, arithmetic operators, comparison operators, logical operators, and bitwise operators can all be used to do mathematical operations, compare values, combine boolean values, and conduct bitwise operations on values.

Intermediate-Level Javascript Interview Questions

  • What is the difference between synchronous and asynchronous programming in JavaScript?

Synchronous programming blocks the execution of code until a task is completed, while asynchronous programming allows the code to continue executing while a task is being completed in the background.

  • What is a Promise in JavaScript?

A Promise in JavaScript is an object that represents a value that may not be available yet, but will be resolved at some point in the future, either with a value or with an error.

  • What is the difference between callbacks and Promises in JavaScript?

Callbacks are functions passed as arguments to another function to be executed when a task is complete, while Promises are objects representing the eventual completion (or failure) of an asynchronous operation, and can be used to handle success and error conditions.

  • How does JavaScript’s async/await work?

Async/await is a syntactic sugar in JavaScript that allows you to construct asynchronous code that looks and behaves like synchronous code.

  • What is the difference between let and const in JavaScript?

let and const are both block-scoped, but let can be reassigned a new value, while const cannot be reassigned once it has been assigned a value.

  • What does JavaScript’s event loop do?

By monitoring the call stack for any pending function calls and the task queue for any waiting events, JavaScript’s event loop enables it to manage several tasks at once.

  • What do JavaScript’s higher-order functions do?

In JavaScript, higher-order functions are those that accept one or more functions as arguments and/or return another function as their output.

  • What do you mean by currying?

Currying is a method used in JavaScript to split up a function that accepts several arguments into several functions that only accept one.

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

Both call and apply methods are used to invoke a function with a specific value of this and arguments are passed as an array (apply) or comma-separated list (call), but the difference is in how the arguments are passed.

  • What is the spread syntax in JavaScript?

The spread syntax in JavaScript is a way to expand the contents of an iterable (like an array or a string) into individual elements.

  • What does JavaScript’s destructuring mean?

JavaScript’s destructuring feature makes it simple and understandable to extract values from objects or arrays and assign them to variables.

  • What does JavaScript’s generator function do?

A generator function in JavaScript allows you to produce a series of values on demand since it may be stopped and restarted during execution.

  • What is the difference between forEach and map methods in JavaScript?

Both forEach and map methods are used to iterate over arrays in JavaScript, but forEach executes a callback function for each element and does not return anything, while map executes a callback function for each element and returns a new array with the results.

  • What is the difference between slice and splice methods in JavaScript?

The slice method returns a shallow copy of a section of an array, whereas the splice method modifies the content of an array by deleting and/or adding components.

  • What is a prototype in JavaScript?

A prototype in JavaScript is an object that serves as a template for creating other objects and contains properties and methods that are shared among all instances of a particular object.

  • What is the difference between prototypal and classical inheritance in JavaScript?

Classical inheritance is a way of creating classes and objects based on a hierarchical class structure, while prototypal inheritance is a way of creating objects by inheriting them directly from other objects.

  • What is the use of the bind method in JavaScript?

The bind method in JavaScript is used to create a new function with a specified value of this, regardless of how the function is called. It is often used in event handlers and callbacks.

  • What are the different ways of creating objects in JavaScript?

There are several ways to create objects in JavaScript, including object literals, constructor functions, object.create() method, and class syntax (introduced in ECMAScript 2015).

  • How do you handle errors in JavaScript?

In JavaScript, you can handle errors using try-catch blocks. The try block contains the code that might throw an exception, and the catch block catches the exception and handles it appropriately. You can also use the throw statement to throw your own exceptions.

  • What are closures in JavaScript, and how are they used?

Closures in JavaScript are functions that have access to the variables of their parent functions, even after the parent functions have returned. They are used to create private variables and methods and to maintain states across function calls.

Advanced Level Interview Questions

  • How does JavaScript handle garbage collection, and what are some best practices for managing memory in JavaScript?

JavaScript uses automatic garbage collection to reclaim memory that is no longer being used by the program. Some best practices for managing memory in JavaScript include avoiding global variables, minimising the use of closures, and avoiding memory leaks caused by circular references.

  • What is hoisting in JavaScript, and how does it work?

Hoisting is the process by which JavaScript moves function and variable declarations to the top of their scope before executing the code. This means that you can call a function or access a variable before it has been declared, although it is best practice to declare variables before using them to avoid confusion.

  • What is the inheritance syntax in JavaScript, and what are some best practices for writing reusable code?

Instead of classes, JavaScript employs prototypal inheritance, in which things inherit from other objects. Composition over inheritance, object composition over class inheritance, and the use of factory methods and mixins are some best practices for producing reusable code in JavaScript.

  • What does JavaScript’s event delegation accomplish, and how does it operate?

By attaching an event listener to a parent element rather than each individual child element, you may use the concept of event delegation. By doing so, you can handle events more effectively and use fewer event listeners across your code.

  • What is a pure function in JavaScript, and what are some benefits of using pure functions in your code?

A pure function is a function that always returns the same output for a given input, and has no side effects outside of the function. This makes them easier to test and reason about and reduces the likelihood of bugs in your code.

  • In JavaScript, what is a higher-order function, and how can it be utilised to produce more modular and reusable code?

A higher-order function accepts another function as input and returns another function as output. This enables you to write more modular and reusable code and is an important component of functional programming in JavaScript.

  • What are some common design patterns in JavaScript, and how can they be used to improve the quality and maintainability of your code?

Some common design patterns in JavaScript include the Module pattern, the Factory pattern, the Observer pattern, and the Singleton pattern. These patterns can be used to improve the quality and maintainability of your code by promoting modularity, encapsulation, and code reuse.

  • What is the distinction between JavaScript’s call, bind and apply methods, and how can you use each of them to change the context of a function?

The call, apply, and bind methods are used to manipulate the context of a function, allowing you to specify the value of the “this” keyword and pass arguments to the function. The main difference between them is the way in which arguments are passed to the function.

  • How does the JavaScript event system function, and how can you implement custom events in your code?

To handle user input, network events, and other sorts of events, the JavaScript event system employs event listeners and dispatchers. With the Event constructor and the dispatchEvent function, you may define custom events in your code.

  • What is a JavaScript web worker, and how can it be used to offload CPU-intensive tasks off the main thread?

A web worker is a background thread of execution that allows you to offload CPU-intensive activities off the main thread. This can improve your program’s responsiveness and prevent the UI from freezing while long-running activities are executed.

  • How can you utilise a web worker in JavaScript to divert the main thread from CPU-intensive tasks?

You may delegate CPU-intensive activities from the main thread to a web worker, a second thread of operation that runs in the background. This can make your software more responsive and stop the UI from stalling while lengthy processes are finished.

  • What is the difference between the ES5 and ES6 versions of JavaScript, and what are some new features introduced in ES6?

ES6 introduced several new features and improvements to the language, including arrow functions, template literals, destructuring, classes, and modules. It also introduced let and const for variable declaration, which have different scoping rules than var. Overall, ES6 made JavaScript more expressive and easier to write and maintain.

  • What are some differences between JavaScript and other programming languages like Java or Python?

JavaScript is a dynamically-typed language that runs in a browser environment and is primarily used for web development. It is also single-threaded and asynchronous by nature, whereas languages like Java or Python are multi-threaded and synchronous. Additionally, JavaScript has a unique event-driven programming model that allows developers to create highly interactive user experiences.

  • What is the difference between mutable and immutable values in JavaScript, and why is it important?

In JavaScript, immutable values are those that cannot be modified after being generated, whereas mutable values can. Immutable values are more predictable and simpler to reason about than mutable values, hence this distinction is significant. A variable’s value can mistakenly change when it has mutable values, which can result in unexpected behaviour and errors.

  • What is the difference between synchronous and asynchronous I/O in JavaScript, and how can they be used to manipulate input and output?

Both synchronous and asynchronous input and output (I/O) operations are possible with JavaScript. Synchronous I/O causes code to block as it waits for an I/O operation to finish, which can make programs sluggish and unresponsive. 

Contrarily, asynchronous I/O permits other code to continue running while the I/O operation is ongoing, which can enhance an application’s performance and responsiveness.

Tips for Acing Your JavaScript Interview on the first attempt

Here are some tips for acing your JavaScript interview on the first attempt:

Brush up on Key Concepts and Principles

Before the interview, make sure you have a solid understanding of the key concepts and principles of JavaScript development, including variables, data types, control flow, functions, and objects.

Practice Coding Problems and Challenges

There are many resources available online to help you practise coding problems and challenges, which will help you build your skills and improve your ability to solve complex problems in real time during the interview.

Prepare for Behavioural Questions

In addition to technical questions, many interviewers will also ask behavioural questions to assess your communication and collaboration skills. Be prepared to answer questions about your work experience, team projects, and problem-solving approach.

Keep Cool and Confident Throughout the Interview Process

Be calm and confident during the interview process, especially if you are asked a challenging question. Take a deep breath, relax, and don’t be hesitant to ask for clarification if necessary.

Follow up after the Interview

After the interview, send a thank you email or letter to your interviewer, expressing your excitement for the opportunity to join their team.

Common Mistakes to Avoid During the Interview Process

These are some common JavaScript interview blunders to avoid:

Not Grasping the Fundamentals

Check that you grasp the essential ideas of JavaScript, such as variables, functions, and loops.

Not Coding on a Frequent Basis

Coding on a regular basis will maintain your abilities sharp and keep you up to date with the latest JavaScript advances.

Overemphasis on Syntax

While syntax is important, it is not the most important aspect of JavaScript. Concentrate on comprehending the logic and structure of the language.

Not Asking Clarifying Questions

If you don’t understand a question or need clarification, don’t be hesitant to ask for more information from the interviewer.

Not Discussing Your Thought Process

As you work through a problem, explain your thought process to the interviewer. This will demonstrate your problem-solving skills and allow the interviewer to better understand your approach.

Not Testing Your Code

Before submitting your solution, test it thoroughly to ensure that it works correctly and efficiently. 

Not Demonstrating Your Knowledge of Best Practices

Show the interviewer that you are familiar with best practices, such as using descriptive variable names, commenting on your code, and organising your code into reusable functions.

Not Showing Enthusiasm

Demonstrate your passion for JavaScript by discussing projects you’ve worked on, open-source contributions you’ve made, or courses and books you’ve read.


In conclusion, JavaScript is a critical programming language used in web development and beyond. If you’re preparing for a JavaScript interview, it’s essential to have a solid understanding of the language’s fundamental concepts, syntax, and best practices. 

This article has covered common javascript interview questions and answers, ranging from basic to advanced, along with their answers to help you prepare for your interview. 

By avoiding common mistakes and demonstrating your problem-solving skills and enthusiasm for the language, you can ace your JavaScript interview and land the job of your dreams.

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