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JavaScript Interview Questions

JavaScript is one of the most widely used programming languages in web development. As a JavaScript developer, facing interview questions is a crucial part of the job-seeking process. This introduction will delve into the significance of JavaScript interview questions and provide insights into how you can prepare effectively for them.

JavaScript is at the core of web development, and acing interviews is crucial. Whether you’re new or intermediate, our list of JavaScript interview questions and answers will help you confidently tackle those interviews.

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Let’s dive into these questions and answers to strengthen your JavaScript knowledge and boost your confidence when facing those critical job interviews.

Q1. What is JavaScript, and what are its key features?

Ans: JavaScript is a high-level, dynamically typed, and versatile programming language primarily used for web development. Its key features include:

  • Client-Side Scripting: JavaScript is primarily used to add interactivity and manipulate web pages within a user’s browser.
  • Dynamic Typing: Variables in JavaScript are dynamically typed, which means their data type can change during runtime.
  • Event-Driven: JavaScript is event-driven, allowing developers to respond to user actions like clicks and input.
  • Single-Threaded: JavaScript is single-threaded, executing one operation at a time, but it can handle asynchronous tasks using callbacks and promises.
  • Cross-Platform: JavaScript is supported by all major browsers, making it a cross-platform language.

Q2. What is the key difference between JavaScript and Java?

Ans: JavaScript and Java are entirely different languages. The key differences include:

  • Language Type: JavaScript is a scripting language, while Java is a full-fledged, object-oriented programming language.
  • Platform: JavaScript runs in a browser, whereas Java applications run on the Java Virtual Machine (JVM) across various platforms.
  • Syntax: While both languages have C-like syntax, they are not interchangeable. JavaScript is used for web development, whereas Java can be used for a wide range of applications.

Q3. Understanding JavaScript Data Types

Ans: JavaScript data types can be categorized into three major groups:

1. Primitive Data Types:

  • Numbers: Used for numeric values.
  • Strings: Used for text data.
  • Boolean: Represents true or false values.
  • Symbol: Introduced in ES6, used to create unique values.

2. Trivial Data Types:

  • Undefined: Represents a variable that has been declared but hasn’t been assigned a value.
  • Null: Represents the intentional absence of any value.

3. Composite Data Types:

  • Objects: Used to store collections of data as key-value pairs.
  • Functions: Represent reusable blocks of code.
  • Arrays: Special objects designed to store ordered lists of data.

Understanding these data types is fundamental to working with JavaScript as they dictate how data is stored, manipulated, and used in your programs.

Q4. Commenting in JavaScript: What Symbols Are Used?

Ans: Comments in JavaScript serve the purpose of providing explanations and annotations in your code, and they are ignored during execution. In JavaScript, there are two main symbols used to represent comments:

1. Double Slash (//): Single-Line Comments

  • Single-line comments are indicated by two forward slashes followed by your comment text.
  • They are ideal for adding brief comments on a single line of code.
  • Example:
// This is a single-line comment

2. Slash with Asterisk (/ … /): Multi-Line Comments

  • Multi-line comments start with a forward slash followed by an asterisk (/) and end with an asterisk followed by a forward slash (/).
  • They allow you to include comments that span multiple lines.
  • Example:
   This is a multi-line comment.
   It can span across several lines.

Using comments effectively is essential for code documentation and collaboration, helping you and others understand the code’s functionality and purpose.

Q5. JavaScript Expression Evaluation: What’s the Result of 3 + 2 + “7”?

Ans: In JavaScript, when you perform the operation 3 + 2 + "7" it’s crucial to understand how data types affect the result. Here’s the breakdown:

  • The first two numbers, 3 and 2, are treated as integers. When you add them together, you get 5.
  • However, when you add a string, “7,” to this result, JavaScript performs type coercion, converting the numeric result (5) to a string.
  • As a result, the final output is not the sum of the numbers; instead, it’s the concatenation of the number 5 with the string “7.”
  • The result is "57", where the numbers and the string are combined to form a new string.

This type coercion in JavaScript can sometimes lead to unexpected results, so it’s essential to be aware of how data types interact in your code.

Q6. What Is the Purpose of the isNaN Function in JavaScript?

Ans: In JavaScript, the isNaN function is utilized to ascertain whether a given value is NaN (Not a number) and belongs to the “Number” type. If the argument is not a number, it returns true; otherwise, it returns false. This is valuable for validating numeric input and handling calculations.

Q7. JavaScript vs. ASP Script Speed in Web Development

Ans: JavaScript is typically faster than ASP Script. JavaScript is executed on the client side, independent of the server, while ASP Script is server-side and reliant on server processing, making JavaScript more responsive in web applications.

Q8. Exploring Negative Infinity in JavaScript

Ans: Negative Infinity in JavaScript is a constant that signifies the lowest possible numeric value. It serves as a representation of a value that is less than any other number. You can create Negative Infinity through custom functions or arithmetic operations, and it’s displayed in JavaScript as “-Infinity.” Understanding this concept is essential for handling extreme numerical scenarios in your code.

Q9. Explain hoisting in JavaScript with an example.

Ans: Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their containing scope during compilation. For instance:

console.log(x); // Outputs 'undefined'
var x = 5;

In this example, the variable x is hoisted to the top of its scope, but the assignment happens later, resulting in undefined.

Q10. Differentiate between let, const, and var in variable declaration.

Ans: Variables can be declared using varlet, or const keywords.

  • var: Used for declaring variables in the global or function scope. It has function-level scope.
  • let: Introduced in ES6, used for block-scoped variables.
  • const: Also introduced in ES6, used for declaring constants with block scope.

Q11. What is a closure in JavaScript? Provide an example.

Ans: A closure is a function that retains access to variables from its containing function even after that function has finished executing.

function outer() {
  let outerVar = 5;
  function inner() {
  return inner;
const innerFunc = outer();
innerFunc(); // Outputs 5

In this example, inner maintains access to outerVar, demonstrating closure.

Q12. Explain the variable scopes in JavaScript.

Ans: In JavaScript, variable scope refers to the visibility and accessibility of variables within a program. There are two primary scopes:

Global Scope: Variables declared in the global scope are accessible from anywhere in the JavaScript code. These global variables are defined outside of any function.

const globalVariable = "I'm a global variable";

function someFunction() {
    console.log(globalVariable); // Accessible

Local Scope: Variables declared inside a function have local scope. They are only accessible within the function in which they are defined.

function localScopeExample() {
    const localVar = "I'm a local variable";
    console.log(localVar); // Accessible

console.log(localVar); // Error, localVar is not defined in this scope

Local variables are confined to the function they are declared in, while global variables can be accessed from any part of the code.

Q13. Explain the purpose and usage of the this keyword in JavaScript.

Ans: The this keyword refers to the current context and can vary based on how a function is called (e.g., in a method, as a constructor, etc.). It allows you to access the properties of the current object.

Q14. Discuss the differences between == and === in JavaScript.

Ans: == is a loose equality operator, while === is a strict equality operator. == compares values after type coercion, while === compares both value and type. For example:

5 == '5'  // true (loose equality)
5 === '5' // false (strict equality)

Q15. How can you define an object in JavaScript?

Ans: JavaScript is inherently object-oriented, and creating objects is a fundamental aspect of its development. Here’s a basic example:

const person = {
    name: 'Alice',
    age: 25

Q16. How do you create an array in JavaScript?

Ans: To construct arrays in JavaScript, you can use the array literal notation. Here are some examples:

const emptyArray = [];

const letters = ['a', 'b', 'c', 'd', 'e'];

Q17. What are the guidelines for naming variables in JavaScript?

Ans: When naming variables in JavaScript, it’s important to follow these conventions:

  1. Avoid Reserved Keywords: Variable names cannot be the same as reserved keywords in JavaScript. For example, you can’t name a variable “var,” “let,” “const,” or any other keyword.
  2. Start with a Letter or Underscore: Variable names must begin with a letter (a-z, A-Z) or an underscore (_) character. They cannot start with a numeric value.
  3. Case Sensitivity: JavaScript variable names are case-sensitive. This means that “myVariable” and “myvariable” are considered different variables.

Here’s an example that illustrates these naming conventions:

// Valid variable names
let userName = "John";
let _counter = 0;

// Invalid variable names
let 123variable = "Invalid"; // Starts with a numeric value
let break = "Invalid"; // Uses a reserved keyword
let MyVariable = "Valid"; // Case-sensitive, but it's best to be consistent

Q18. What is the difference between Function declaration and Function expression?

Ans: A function declaration defines a named function, which is hoisted and can be called before it’s declared. A function expression defines a function as an anonymous value within an expression.

DifferenceFunction DeclarationFunction Expression
Declaration TypeDeclared as a separate statement within the main JavaScript codeCreated inside an expression or some other construct
HoistingCan be called before the function is definedCreated when the execution point reaches it; can be used only after that
Readability and OrganizationOffers better code readability and better code organizationUsed when there is a need for a conditional declaration of a function
Examplefunction abc() {
return 5;
var a = function abc() {
return 5;

Q19. Understanding Cookies in JavaScript: Creation, Reading, and Deletion of Cookies

Ans: Cookies are small pieces of data sent from a website and stored in a user’s web browser. They serve to remember information and track browsing activity. In JavaScript:

  • Creation: To create a cookie, use document.cookie = "key1=value1; key2=value2; expires=date";.
  • Reading: Reading a cookie is straightforward. Use document.cookie, which contains the cookie information, and split it into key-value pairs as needed.
  • Deletion: To delete a cookie, set an expiration date and provide the correct cookie path, such as:
    function delete_cookie(name) {
    document.cookie = name + “=; Path=/; Expires=Thu, 01 Jan 1970 00:00:01 GMT;”;

Q20. Differentiating Between Undefined and Undeclared Variables in JavaScript

Ans: In JavaScript, it’s essential to understand the distinction between “undefined” and “undeclared” variables:

Undefined Variables:

  • These occur when a variable has been declared but hasn’t been assigned any value.
  • Undefined is not a keyword but a state where a variable exists, but it lacks a value.

Undeclared Variables:

  • These occur when you attempt to access a variable that hasn’t been initialized or declared using the var or const keyword.
  • When you use the typeof operator on an undeclared variable, it results in a runtime error with the return value “undefined.”
  • The scope of undeclared variables is always global.

Understanding these differences is crucial for effective variable management in JavaScript.

Q21. Understanding the Concept of NULL in JavaScript

Ans: In JavaScript, “NULL” represents the absence of a value or object. It’s akin to an empty value or object in the language. Recognizing the nature of NULL is vital for managing data and variables effectively in JavaScript.

Q22. The Purpose of a Prompt Box in JavaScript

Ans: A prompt box in JavaScript is a dialog box that can display an optional message, asking the user for input, typically a text value. It’s commonly used when a user needs to provide information or input before proceeding on a webpage. The prompt box returns a string containing the text entered by the user or null if the input is canceled. Understanding how to use prompt boxes is valuable for interactive web applications.

Q23: Timers in JavaScript: How They Work and Their Potential Limitations

Ans: Timers in JavaScript, facilitated by functions like setTimeout, setInterval, and clearInterval, are essential for executing specific code at designated times or for repetitive tasks. For example, if a page needs to display an alert message like “Time’s up” after a 2-minute delay, you can achieve this using setTimeout. This method calls a function or evaluates an expression after a specified number of milliseconds. However, there are potential drawbacks to consider, such as managing resource usage, potential delays in execution, and ensuring efficient code to avoid performance issues. Understanding timer functions and their limitations is crucial for creating responsive and efficient JavaScript applications.

Q24. ViewState vs. SessionState in Web Development

Ans: In web development, it’s important to distinguish between “ViewState” and “SessionState”:


  • ViewState is page-specific and pertains to a single web page in a session.
  • It’s used to retain state or data on a specific page during postbacks, ensuring data persistence within that page’s lifecycle.


  • SessionState is user-specific and can store and access data across multiple web pages within a session.
  • It provides a broader scope for managing and preserving user-specific information and data throughout their interaction with the website.

Understanding these differences is crucial for effectively managing data and user interactions in web applications.

Q25. Understanding Global Variables in JavaScript and Their Challenges

A: Global variables in JavaScript are those defined outside of functions. These variables have a global scope, making them accessible by any function without needing to pass them as parameters.


let nickName = "Jared"; // Global Variable

function myFunction() {
    document.getElementById("clouds").innerHTML = typeof nickName + "- " + "My nick name is " + nickName;

document.getElementById("Clouds").innerHTML = typeof nickName + "- " + "My nick name is " + nickName;

However, relying heavily on global variables can introduce challenges. Debugging and testing code that depends on global variables can be problematic. These variables may lead to unexpected behavior and make it challenging to track issues and maintain clean, organized code. Hence, it’s advisable to minimize the use of global variables for efficient development and easier debugging.

Additional Resource

Explore a variety of valuable resources to help you prepare for your JavaScript interviews, from detailed explanations to interactive coding challenges.

1. JavaScript Interview Questions on MDN Web Docs : MDN Web Docs provides a comprehensive list of JavaScript interview questions, complete with explanations and examples. This authoritative resource ensures you’re well-prepared for any interview.

2. Interviewing.io – JavaScript Interview Tips : Interviewing.io features recorded mock interviews where experienced engineers tackle real interview questions. Watch and learn from these interviews to refine your own interview technique.


As we wrap up this comprehensive guide on JavaScript interview questions, it’s important to recap the key takeaways and provide some final tips to help you excel in your upcoming JavaScript interviews.

A. Recap of Key Takeaways

Throughout this guide, we’ve covered various aspects of JavaScript interviews, including core concepts, DOM manipulation, frameworks, error handling, ES6+ features, best practices, and sample questions. Here are the key takeaways:

  1. Understanding JavaScript Fundamentals: A strong grasp of JavaScript’s core concepts is essential. This includes variables, data types, functions, and scope.
  2. DOM Manipulation: Mastering the Document Object Model (DOM) and understanding how to manipulate it is crucial for web development.
  3. Framework Familiarity: If you’re interviewing for positions involving JavaScript frameworks, ensure you’re well-versed in the fundamentals of the specific framework.
  4. Error Handling and Debugging: Be prepared to handle errors and demonstrate your debugging skills using various tools.
  5. ES6+ Features: JavaScript has evolved with ES6 and beyond. Familiarize yourself with new features like arrow functions, classes, modules, and more.
  6. Best Practices: Adhering to JavaScript best practices is essential for writing clean, efficient, and secure code.
  7. Practice, Practice, Practice: Solve sample JavaScript interview questions to refine your problem-solving skills and gain confidence.

B. Final Tips for Acing Your JavaScript Interview

To ensure success in your JavaScript interview, consider the following final tips:

  1. Stay Calm and Confident: Maintain composure during the interview. Confidence in your abilities is key.
  2. Communication Skills: Clearly articulate your thought process when answering questions. Effective communication is highly valued.
  3. Ask Questions: Don’t hesitate to ask clarifying questions when faced with ambiguous or complex problems. It demonstrates your analytical skills.
  4. Mock Interviews: Conduct mock interviews with a friend or mentor to simulate the interview experience.
  5. Research the Company: Learn about the company and its specific needs. Tailor your responses to show how you can contribute to their goals.
  6. Continuous Learning: Keep up with the latest developments in JavaScript and web development. Interviewers may inquire about new trends and technologies.
  7. Portfolio Showcase: If you have personal projects or contributions to open-source projects, highlight them to demonstrate your practical experience.
  8. Follow Up: After the interview, send a thank-you note to express your gratitude and continued interest in the position.

With these key takeaways and final tips in mind, you’re well-prepared to tackle JavaScript interviews with confidence and increase your chances of success. Good luck!

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