Top 25 JavaScript Interview Questions and Answers in 2024

Editorial Team

JavaScript Interview Questions and Answers

JavaScript is among the widely used web development languages. If you are preparing for a JavaScript interview, the following are the commonly asked question that you can expect in your next interview. We have provided a sample answer for each interview question to help you know how best to respond.

1. Please Explain The Various Javascript Data Types

Data types in Javascript explains the different kind of data that the users stores in the database. These types include:

  • Strings data- String data represent a sequence of characters(textual data). They are developed using single quotes or double quotes surrounding single or multiple characters.
  • The number data- This data type is used to indicate negative or positive numbers with a decimal place as well as those without decimal numbers. They also represent numbers with exponential notation.
  • The Boolean data:  The Boolean data is used to store values such as yes, no, on, off, etc. The data can only hold two values.
  • The undefined data: This type of data represents a variable that has an undefined value. This data can only have one value(undefined value).
  • The null data- is a type of data that stores data with no value. The data has only one value.
  • The object data- this data allows the user to store collections of data. It contains key-value pair.
  • The Array data- This type of data stores multiple values in a single variable.

2. Highlight And Explain Various Features Of Javascript

As one of the popular programming languages,  JavaScript has numerous features such as :

  • It allows functional style. JavaScript allows a functional approach such that even objects are generated from the constructor functions. Every constructor function represents a very unique object type.
  • It supports dynamic typing. This means that variables are defined depending on the stored value.
  • JavaScript is a lightweight scripting language. This means that JavaScript has a limited set of libraries since it’s not a general-purpose language.
  •  It is platform-independent. This means that it is more portable and allows the user to write the script anytime and anywhere.
  • JavaScript is an interpreted language. The script written is processed line by line and interpreted by a JavaScript interpreter( an inbuilt component of the web browser).

3. How Is Javascript Different From Java?

The JavaScript programming language isn’t part of the Java platform. JavaScript is an OOP scripting language while Java is an OOP programming language. Java script code is all in the text while java code requires to be compiled. Additionally,  JavaScript code is run on a browser while java generates applications that are run in a browser or virtual machine.

4. What Are The Scopes Of A Variable In Javascript?

 JavaScript variables have got  different scopes including:

  • Global Scope- This refers to any variable that is declared outside a function. It can also be described as a variable that can be accessed at any place in the program. It is defined using keywords such as Const, let, and var.
  • Local Scope –  a variable is said to be a local scope if it is declared inside a function.
  • Block Scope-this means that you cant access those variables that have been declared inside a specific block for the outside of the block.
  • Function scope-this means that a user cant access the variables that have been defined on the inside of a function from outside of another function.

5. Describe The Life Of A Variable In Javascript

The lifespans of a variable are determined by the scope of that variable. This means that the lifespans begin when a variable is declared.  For instance, a local variable exists up to when a function is under implementation. A global variable exists up to when a web browser is shut down or until when the user shut down the program.

6. Explain  How To Debug A Javascript Code

Most of the latest browsers such as firefox and chrome contain an inbuilt debugger. This debugger can be accessed by a user at any time. He or she needs to press the right key mostly the F12. Another way of debugging a javascript code is by using javascript applications such as Atom, sublime text, visual studio code, etc.

7. Could You Tell Us About The Various Types Of Error Constructors Supported By Javascript?

The error constructor creates an error object. Javascript offers support for 7 error constructors including:

  • URIError– generates an error that is created when invalid parameters are passed to the encodeURI() or decode URI()
  • SyntaxError- Generates  an error regarding an error that occurs while parsing code in eval()
  • EvalError- generate an error instance about the global function eval()
  • ReferenceError- generates an error instance about de-referencing an invalid reference.
  • TypeError – Generates an error instance about a  variable or parameter, not of a valid type.
  • RangeError – generates an error instance about a parameter or numeric variable that is outside of its valid range.
  • InternalError- Generates an error instance about an internal error in the Javascript engine

8. Explain The Difference Between Function Declaration And Function Expression

Both Function Declaration And Function Expression use keyword function. However, they also have differences. A function expression does not require a function name while a function declaration must have a function name. Function declaration doesn’t need a variable assignment but function expressions require a variable assignment. Function expressions don’t require to be hoisted while function declarations are hoisted. Function declarations are implemented before any other code while function expressions are implemented only when the program interpreter has reached the code line.

9. What Are The Advantages Of Javascript Over Other Web Technologies?

Quick feedback: This means that the user doesn’t require a web page to reload while running JavaScript.

Enhanced interaction: JavaScript adds an interface to static web pages thus making them react to users’ inputs.

Frameworks: JavaScript has numerous libraries and frameworks that can be used to create web applications and games of all kinds. Rich user interface: JavaScript makes the UI of web applications appear and feel better.

10. Kindly, Explain The Ways Of Adding Javascript Code In An HTML File?

There are two main ways of embedding JavaScript code including:

  • JavaScript code can be written within the script tag in the same HTML file especially when the user requires a few lines of scripting on a web page.
  •  A user can also import a JavaScript source file into an HTML document. All the scripting abilities to a web page without cluttering the code.

 11. How Do You Empty An Array In Javascript?

There are several methods of emptying an array in JavaScript. These include:

  • You can assign an array to a new empty array. A code works perfectly if the user has no reference to the preceding array.
  • Setting its length to zero.
  • Removing all its element by using the splice method
  • Remove every element of the array one after the other using the pop method and while loop.

12. What Is The Strict Mode In Javascript?

Strict mode brings in more stringent error-checking in JavaScript code. The user can support strict mode by putting ‘use strict” before the JavaScript code. In strict mode, all values must be declared explicitly. This means that variables cannot be stored as a read-only property.

13. How Do You Remove Duplicates From A Javascript Array?

You can either remove duplicates from a JavaScript array by using the filter method or by using the For Loop. When using the For Loop, the user uses an empty array to store all the repeating methods. On the other side, when using the filter method, you require the current element, the index of the current element, and the array.

14. How Would You Read A Cookie?                                                                  

Reading a cookie with Javascript is not a complex thing. A user uses the document cookie string. Usually, this document has the cookies that the user has just created. That string retains a list of name variable pairs that are separated using a semicolon. What is indicated as a name refers to the cookie’s name while the one indicated value refers to the value of the cookie. Another method of reading a cookie is by using the split method. Here the cookie is broken into values and keys.

15. What Is Memoization?

Memoization is a particular caching that is normally used in dynamic can also be seen as an optimization method in caching that emerges when the same set of arguments comes out in the same performant web applications and output

16. What Is The Rest Parameter And Spread Operator?

The rest operator enables a user to call a function with any variable of arguments and later access those arguments as an array. This operator enables the user to destructor objects or arrays. On the other hand, the spread operator enables the user to expand an enterable-like array into its individual elements.

17. What Is The Use Of Promises In Javascript?

The promise represent an object in JavaScript that indicates the failure or completion of a user task. A promise can be in three states rejected, fulfilled, or pending. While using promise in JavaScript the user can handle the control flow of upcoming data with ease. Also, a user can assign callback functions to the promise if there is fulfillment or rejection of the promise.

18. Why Do We Use Callbacks?

Callbacks ensure that a function does not run before completing a task but instead runs immediately after the task has been completed. Callbacks help a user to create asynchronous JavaScript code. This keeps the user safe from errors and other problems.

19. Highlight Some Of The Ways Of Defining A Variable In Javascript

Javascript can be defined in three ways including:

Const-its are used to define the value in the variable. It is invariable throughout the program and cant be altered at a later time.

Var- it is used to define a value and variable that can be modified later in the javascript code.

Let-this means that the value can be modified later in the javascript code.

20. What Are The Disadvantages Of Using Innerhtml In Javascript?

The disadvantages of using innerHTML are:

  • Content is replaced anytime anywhere
  • The whole innerHTML content is broken down and created into elements. Thus it is much slower.
  • The innerHTML  doesn’t allow validation. Thus, it is easy to insert a valid and reparsed HTML in the document and then break it.
  • It is not usable in the form of ‘appending to innerHTML’
  • Even if the user uses +=like, the old content is still replaced by HTML

21. What Are Some Of The Javascript Frameworks And Their Uses?

Javascript  has different frameworks such as:

React- This is a framework designed by Facebook that eases the process of creating interactive UIs. It can be referred to as a base of react-native. React is a one-way data flow framework. It enables developers to see changes including how they are applied.

Angular-  this framework is maintained by Google to handle common problems while creating single-page applications. Developers use this framework to leverage HTML vocabulary on a dynamic web page.

Node- this framework works as a runtime environment. It is a backend side development of a web application. It helps in server-side scripting and command-line tools.

Vue- this framework focuses only on the view layer. This framework uses an MVVM (model-view-ViewModel) architectural pattern. Ember- This is a framework that uses component service patterns. Embe framework can be used in different parts of the user applications and can be very good for persistent states.

22. State Some Of The Roles You Expect To Be Performing Should You Get This Job

I reviewed the job posting for this role. I am therefore aware of the roles that await me such as:

  • Modifying software to fix any error, adapting software to new hardware, improving the performance of the software, and upgrading interfaces
  • Facilitating software testing as well as the validation procedures
  • Facilitating system programming as well as documentation development
  • Preparing reports about programming project activities, status, or specifications
  • Consulting with users on project proposals or status
  • Deliberating with project managers to acquire information on capabilities or limitations
  •  Analyzing data to recommend  the right systems or modifications of a software
  • Designing and creating software systems using mathematical models and scientific analysis to measure and predict outcomes as well as design consequences
  • Collaborating with other engineers to examine software-hardware interfaces, develop specifications, and performance conditions

23. Describe Your Experience Relating To The Role You Are Seeking

I have been a software developer for 6 years now. During this time, I have gained a lot of experience in building software by writing codes, modifying software, improving the performance of software, upgrading interfaces, etc. I have also gained experience in testing and validating procedures. With my extensive knowledge of programming languages,  I can work with users on technical issues such as software design and maintenance. Besides, I have acquired a lot of skills necessary for this role including the ability to work under pressure, communication skills, organizational, creativity, and analytical skills. Thus, I believe that should you give me this opportunity, I will exceed your expectations.

24. Why Do You Feel You Are The Most Suited For This Role?

 I possess very strong analytical, organizational, and decision-making skills.  I hold a bachelor’s degree in computer science. I have worked for four years as a software developer where I gained experience working with different coding languages such as JSON, Ajax, HTML, and Python JQuery. My solid understanding of software development has made me competent in this field. I am flexible in the workplace. Besides, I have strong communication, organization, team working, analytical, and problem-solving skills. I can also work under pressure and tolerate different cultural differences. Given a chance, therefore,  I  will use all my experience, skills, education, and strong qualities to ensure that I exceed your expectation for this role.

25. Highlight Differences Between Event Bubbling And Event Capturing

Event Bubbling – This is a process that begins with capturing the event from the innermost component before propagating it to the outermost component. Whenever an event takes place on a component, the event handlers begin running on it on its parent before proceeding to its parent and then to the child component and later to its other ancestors.

Event Capturing –This is the process that begins with capturing the event from the outermost component before propagating it to the innermost component. It is the opposite of event bubbling. The event begins from its parent component to its child component.


Javascript is becoming popular every day. However, the company only hires the most qualified candidates. You will therefore need to prepare very well for you to crack the interview and outshine other candidates. It would also be wise to go through their website to get more insights into their working culture