Top 50+ JavaScript Interview Questions and Answers

LISTEN TO THE JAVASCRIPT FAQs LIKE AN AUDIOBOOK

JavaScript is a valuable programming language for web development. It is a client-side language that supports the creation of interactive web pages and web applications to enhance user experience. According to Github, it is amongst the top most coding languages. Therefore, most companies seek developers with extensive JavaScript knowledge. If you are an experienced developer or a coding aspirant, learning JavaScript interview questions is crucial to get a job.

To give you a comprehensive knowledge of JavaScript and prepare you for your next job interview, we have prepared a list of commonly asked JavaScript interview questions and answers.

The Importance of JavaScript

Every web developer must know JavaScript thoroughly to create dynamic and responsive web pages. Understanding JavaScript can also help optimize the website for search engines, improving the website's speed and performance. It is also used in server-side programming, game, and mobile app development. Due to its adaptability and ease of use, JavaScript has become a highly popular choice among developers.

The features of JavaScript help you understand what it's capable of.

  • Simple to comprehend and learn
  • Being an interpreted language, it is fast
  • JavaScript integrates with other coding languages
  • Lightweight, client-side language
  • Supports dynamic typing
  • Follows object-oriented programming
  • Portable due to platform independence

Even though JavaScript is an easy language, you must prepare top JavaScript interview questions before appearing for an interview.

How to prepare for JavaScript Interview?

If you plan to work as a JavaScript developer, then you need to prepare yourself for the interview process. The interviewers may ask you questions on basic and advanced topics to test your knowledge and skills. Practicing JavaScript interview questions and answers is essential to starting a career in JavaScript. Common asked JavaScript questions for interviews include the basics of the language, such as what JavaScript is used for, features of JavaScript, the data types, and the different types of operators used in JavaScript.

Advanced JavaScript interview questions include topics like JavaScript compiler, function code execution, lexical and dynamic scope, IIFE Pattern, explicit binding, closures, etc. Answering these questions demonstrates a deep understanding of the language and its capabilities.

As preparing for a JavaScript interview requires a structured approach, here are some steps and tips to help you get ready:

  • Review JavaScript Fundamentals: Ensure you have a solid understanding of core JavaScript concepts like variables, data types, operators, functions, loops, conditionals, and objects.
  • DOM Manipulation: Familiarize yourself with Document Object Model (DOM) manipulation, as it is crucial for interacting with web pages.
  • Asynchronous JavaScript: Understand asynchronous programming, callbacks, promises, and async/await, as many web applications rely heavily on asynchronous operations.
  • ES6+ Features: Get familiar with newer JavaScript features introduced in ECMAScript 6 and beyond, like arrow functions, classes, let/const, spread/rest operators, etc.
  • Closures and Scope: Understand how closures work and be prepared to explain scope-related concepts.
  • Prototypes and Inheritance: Learn about JavaScript's prototype-based inheritance model.
  • Functional Programming: Be aware of functional programming concepts like higher-order functions, map, filter, reduce, etc.
  • Common Libraries and Frameworks: Familiarize yourself with popular JavaScript libraries and frameworks like React, Angular, or Vue.js, depending on the specific job requirements.
  • Practice Coding: Practice coding exercises on websites like LeetCode, Codewars, or HackerRank. It will help you become more confident in writing JavaScript code under pressure.
  • Review Past Projects: Go through any JavaScript projects you have worked on in the past. Be ready to explain the code you've written, the challenges you faced, and how you solved them.
  • Interview-Specific Topics: Based on the job description or the company you are interviewing with, focus on specific topics like front-end development, back-end integration, API consumption, etc.
  • Whiteboard Practice: Practice solving problems on a whiteboard or a piece of paper. This is a common interview format, so being comfortable with it can be beneficial.
  • Ask Questions: Don't hesitate to ask your interviewer questions if you need clarification on a problem or want to discuss your thought process.
  • Mock Interviews: Conduct mock interviews with your friends or colleagues to simulate the interview experience and receive feedback on your performance.

Careers after preparing JavaScript Interview Questions

Preparing technical interview questions can be a valuable exercise not just for interviewees but also for those looking to enhance their knowledge and expertise in JavaScript. Once you've honed your skills in JavaScript and gained a good understanding of the language, you can explore various career opportunities in the tech industry. Here are some potential career paths you can consider:

  • Front-end Developer: As a front-end developer, you'll work on building user interfaces and interactive elements for websites and web apps using CSS, HTML, and JavaScript. You'll be responsible for ensuring a seamless user experience and translating design mockups into functional code.
  • Full-stack Developer: The full stack professional is proficient in both front-end and back-end technologies. In addition to JavaScript, you'll need to learn back-end languages and frameworks, such as Node.js, Express.js, and databases like MongoDB or MySQL. Full-stack developers can handle the complete development process, from the UI to the server-side logic.
  • Web Developer: While working as a web developer, your core responsibility will be to create and maintain websites, web applications, and web services. JavaScript is a major technology in web development, and having expertise in it will make you highly marketable in this field.
  • Software Engineer: As a software engineer, you can work on developing various software applications, including desktop applications, mobile apps, and backend systems. JavaScript is commonly used for building both front-end and back-end solutions, making it a valuable skill for software engineers.
  • Teaching and Training: After preparing JavaScript interview questions, you can leverage your knowledge and experience to become a JavaScript instructor or tutor, helping others learn and grow their programming skills.
  • Mobile App Developer: If you're interested in mobile development, you can focus on frameworks like React Native or Flutter to build cross-platform mobile applications using JavaScript.
  • UI/UX Designer: Understanding JavaScript can be advantageous for UI/UX designers, as it allows you to create interactive and dynamic user interfaces to demonstrate your design concepts effectively.
  • Game Developer: JavaScript is widely used to develop browser-based games and game engines. If you're passionate about gaming, you can explore this area and work on creating interactive gaming experiences.
  • DevOps Engineer: DevOps engineers are responsible for automating and streamlining the development, testing, and deployment processes. Knowledge of JavaScript can be useful for managing and configuring various DevOps tools and pipelines.

Now let's have a look at our compiled list of the most common JavaScript interview questions 2023 that reinforce your JavaScript knowledge and prepare you for the interviews. If you are looking for some additional interview preparation guidance, feel free to reach out to our placement experts who can help you to build your tech career from scratch.

Answer:

JavaScript is a lightweight, interpreted programming language with object-oriented capabilities that allows you to build interactivity into otherwise static HTML pages. The general-purpose core of the language has been embedded in Netscape, Internet Explorer, and other web browsers.

Answer:

Following are the major differences between Java & JavaScript:

  • Java is an OOP programming language. JavaScript is an OOP scripting language.
  • It creates applications that run in a virtual machine or browser. The code is run on a browser only.
  • Java code needs to be compiled. JavaScript code are all in the form of text.

Answer:

The data types supported by JavaScript are:

  • Undefined
  • Null
  • Boolean
  • String
  • Symbol
  • Number
  • Object

Answer:

Following are the features of JavaScript:

  • It is a lightweight, interpreted programming language.
  • It is designed for creating network-centric applications.
  • It is complementary to and integrated with Java.
  • It is an open and cross-platform scripting language.

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:

The JavaScript this keyword refers to the object it belongs to. This has different values depending on where it is used. In a method, this refers to the owner object and in a function, this refers to the global object.

Answer:

A callback is a plain JavaScript function passed to some method as an argument or option. It is a function that is to be executed after another function has finished executing, hence the name ‘call back‘. In JavaScript, functions are objects. Because of this, functions can take functions as arguments, and can be returned by other functions.

Answer:

The type of operator is used to get the data type of its operand. The operand can be either a literal or a data structure such as a variable, a function, or an object. It 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.

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”;

Answer:

The three possible ways of defining a variable in JavaScript are:

  • Var – The JavaScript variables statement is used to declare a variable and, optionally, we can initialize the value of that variable. Example: var a =10; Variable declarations are processed before the execution of the code.
  • Const – The idea of const functions is not allow them to modify the object on which they are called. When a function is declared as const, it can be called on any type of object.
  • Let – It is a signal that the variable may be reassigned, such as a counter in a loop, or a value swap in an algorithm. It also signals that the variable will be used only in the block it’s defined in.

Answer:

Undefined means a variable has been declared but has not yet been assigned a value. On the other hand, null is an assignment value. It can be assigned to a variable as a representation of no value. Also, undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

Answer:

Event bubbling is a way of event propagation in the HTML DOM API, when an event occurs in an element inside another element, and both elements have registered a handle for that event. With bubbling, the event is first captured and handled by the innermost element and then propagated to outer elements. The execution starts from that event and goes to its parent element. Then the execution passes to its parent element and so on till the body element.

Answer:

NaN is a short form of Not a Number. Since NaN always compares unequal to any number, including NaN, it is usually used to indicate an error condition for a function that should return a valid number. When a string or something else is being converted into a number and that cannot be done, then we get to see NaN.

Answer:

A prompt box is a box which allows the user to enter input by providing a text box. The prompt() method displays a dialog box that prompts the visitor for input. A prompt box is often used if you want the user to input a value before entering a page. When a prompt box pops up, the user will have to click either “OK” or “Cancel” to proceed after entering an input value.

Answer:

There are three types of errors:

  • Load time errors: Errors that come up when loading a web page, like improper syntax errors, are known as Load time errors and generate the errors dynamically.
  • Runtime 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 with a different operation.

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 ‘every time the main function is invoked.

Answer:

JavaScript escape characters enable you to write special characters without breaking your application. 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.

Answer:

Hoisting is the default behavior of JavaScript where all the variable and function declarations are moved on top. In simple words, we can say that Hoisting is a process in which, irrespective of where the variables and functions are declared, they are moved on top of the scope. The scope can be both local and global.

Answer:

Temporal Dead Zone is a behavior that occurs with variables declared using let and const keywords before they are initialized.

Answer:

BOM is the Browser Object Model where users can interact with browsers that is a window, an initial object of the browser. The window object consists of a document, history, screen, navigator, location, and other attributes. Nevertheless, the window’s function can be called directly as well as by referencing the window.