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Top 20 JavaScript Interview Questions and Answers for 2025

Introduction

JavaScript is one of the most popular programming languages, especially for web development. As of 2025, it continues to be essential for frontend development, and with Node.js, it’s also widely used for backend development. Preparing for a JavaScript interview requires a solid understanding of both basic and advanced concepts. In this blog post, we’ll cover the top 20 JavaScript interview questions that are likely to be asked in 2025, along with detailed answers to help you ace your interview. Whether you’re a fresher or an experienced developer, these questions will provide you with the in-depth knowledge needed to succeed.

Before we dive into the questions, if you’re looking to enhance your JavaScript skills or prepare for interviews, consider signing up for our free courses or getting the latest course updates.

Top 20 JavaScript Interview Questions and Answers

1. What is JavaScript, and what are its main features?

JavaScript is a high-level, interpreted programming language that is one of the core technologies of the World Wide Web. It is used to create interactive effects within web browsers. JavaScript was first developed in 1995 by Brendan Eich while he was working at Netscape Communications Corporation. Its main features include:

Dynamic Typing: JavaScript is a dynamically typed language, meaning you don’t need to specify the data type of a variable when you declare it.
First-Class Functions: Functions are treated as first-class citizens in JavaScript, meaning they can be assigned to variables, passed as arguments to other functions, and returned from functions.
Prototype-Based Inheritance: JavaScript uses prototypes for inheritance, which allows objects to inherit properties and methods from other objects.
Asynchronous Programming: JavaScript supports asynchronous programming through callbacks, promises, and async/await, which is crucial for handling operations like network requests without blocking the execution.
Event-Driven: JavaScript is event-driven, meaning it responds to events such as user actions (clicks, form submissions) or network events.
Cross-Platform: JavaScript can run on any platform that has a compatible browser or runtime environment like Node.js.

JavaScript is essential for web development, enabling the creation of dynamic and interactive web pages. To dive deeper into JavaScript fundamentals, check out our web development course.

2. What is the difference between JavaScript and Java?

JavaScript and Java are two distinct programming languages, despite their similar names. Here are the key differences:

Purpose: Java is a general-purpose programming language used for developing a wide range of applications, including desktop, web, and mobile apps. JavaScript is primarily used for web development, specifically for creating interactive effects within web browsers.
Syntax: While both languages share some syntactic similarities (e.g., use of curly braces, semicolons), they have different syntaxes. For example, Java requires variables to be declared with a type, while JavaScript does not.
Typing: Java is statically typed, meaning variable types are known at compile time. JavaScript is dynamically typed, with types determined at runtime.
Execution: Java code is compiled into bytecode, which is then executed by the Java Virtual Machine (JVM). JavaScript code is interpreted by the browser’s JavaScript engine.
Object-Oriented Features: Both languages support object-oriented programming, but Java is class-based, while JavaScript is prototype-based.
Use Cases: Java is used for enterprise applications, Android app development, etc. JavaScript is used for web development, with frameworks like React, Angular, and Vue.js, and also for server-side development with Node.js.

3. What are the different data types in JavaScript?

JavaScript supports several data types, which can be categorized into primitive and non-primitive types.

Primitive Data Types:

Number: Represents numeric values, both integers and floating-point numbers. For example, 42, 3.14.
String: Represents sequences of characters. Strings can be defined with single quotes, double quotes, or backticks (for template literals). For example, “Hello”, ‘World’, `Template literal`.
Boolean: Represents logical values, either true or false.
Undefined: Represents an uninitialized variable. For example, let x; will have the value undefined.
Null: Represents no value or no object. It is often used to indicate the intentional absence of any object value.
BigInt: Introduced in ES2020, BigInt represents whole numbers larger than 2^53 – 1 that are too big for the Number type.
Symbol: Represents unique and immutable values, often used as keys for object properties.

Non-Primitive Data Types:

Object: Represents a collection of key-value pairs. Objects can be created using object literals {} or with the new Object() constructor.
Array: A special type of object that represents an ordered collection of values. Arrays can be created using array literals [] or with the new Array() constructor.
Function: Functions are objects in JavaScript and can have properties and methods. They are defined using the function keyword or function expressions.

4. Explain the concept of hoisting in JavaScript.

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during the compilation phase. This means that regardless of where variables and functions are declared, they are processed as if they were declared at the top of their scope.

There are two types of hoisting:

Variable Hoisting: When you declare a variable using var, it is hoisted to the top of its scope, but its initialization (assignment of value) is not hoisted. So, if you try to access a variable before it is declared, you will get undefined, not an error.

				
					 console.log(x); // undefined
var x = 10;
 This is equivalent to:

 var x;
console.log(x); // undefined
x = 10;

Function Hoisting: Function declarations are fully hoisted, meaning both the declaration and the initialization are moved to the top. So, you can call a function before it is declared.

				
					 foo(); // "Hello"
function foo() {
    console.log("Hello");
}

This works because the function declaration is hoisted entirely.
However, function expressions are not hoisted in the same way. Only the variable is hoisted, not the function assignment.

				
					bar(); // TypeError: bar is not a function
var bar = function() {
    console.log("Hi");
};

In this case, bar is hoisted, but its assignment is not, so calling bar() before declaration results in an error.

Note that let and const declarations are also hoisted, but they are not initialized until their definition is evaluated. Accessing them before their definition results in a ReferenceError due to the temporal dead zone (TDZ).

5. What is the difference between null and undefined in JavaScript?

Undefined: Represents an uninitialized variable or a property that does not exist. It is the default value for variables that have not been assigned a value. For example, let x; will have the value undefined.
Null: Represents no value or no object. It is often used to indicate the intentional absence of any object value. For example, let y = null; explicitly sets y to have no value.

Key Differences:

Feature	Null	Undefined
Intent	Intentional absence of value	Unintentional absence of value
Type	object (historical quirk)	undefined
Usage	Used to clear or reset a value	Indicates unassigned variable

Example:

				
					let x;
console.log(x); // undefined
let y = null;
console.log(y); // null

6. What are closures in JavaScript?

A closure is a function that has access to its own scope, the outer function’s scope, and the global scope, even after the outer function has finished executing. Closures are created when an inner function is defined inside an outer function and has access to the outer function’s variables.

Example:

				
					function outer() {
    let count = 0;
    return function inner() {
        count++;
        console.log(count);
    };
}

const counter = outer();
counter(); // 1
counter(); // 2

In this example, inner has access to count from outer, even after outer has finished executing. This is a closure.

Closures are useful for creating private variables and maintaining state across function calls. To learn more about closures, explore our web development course.

7. Explain callback functions in JavaScript.

A callback function is a function passed as an argument to another function and executed after some operation has been completed. Callbacks are commonly used in asynchronous programming, such as handling events or making API requests.

Example:

				
					function fetchData(url, callback) {
    // Simulate an asynchronous operation
    setTimeout(() => {
        const data = "Data from " + url;
        callback(data);
    }, 1000);
}

fetchData("example.com", (data) => {
    console.log(data); // "Data from example.com"
});

Here, fetchData takes a URL and a callback function. After a delay (simulating an async operation), it calls the callback with the result.

8. What are promises in JavaScript, and how do they work?

A promise is an object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value. Promises provide a cleaner way to handle asynchronous code compared to callbacks.

A promise has three states:

Pending: The initial state; neither fulfilled nor rejected.
Fulfilled: The operation completed successfully.
Rejected: The operation failed.

You can create a promise using the Promise constructor:

				
					const promise = new Promise((resolve, reject) => {
    // Asynchronous operation
    setTimeout(() => {
        const success = true;
        if (success) {
            resolve("Success!");
        } else {
            reject("Error!");
        }
    }, 1000);
});

promise
    .then((result) => console.log(result)) // "Success!"
    .catch((error) => console.error(error)); // "Error!"

Promises can be chained using .then() for success and .catch() for errors.

9. What is the purpose of the setTimeout() function in JavaScript?

The setTimeout() function is used to execute a specified function or piece of code after a specified delay (in milliseconds). It is commonly used for scheduling tasks or creating delays in asynchronous operations.

Example:

				
					console.log("Start");
setTimeout(() => {
    console.log("This runs after 2 seconds");
}, 2000);
console.log("End");

Output:
Start
End
This runs after 2 seconds

setTimeout() is non-blocking, meaning it does not pause the execution of the code; instead, it schedules the callback to run later.

10. How can you check if an array includes a certain value in JavaScript?

You can use the includes() method to check if an array contains a specific value. It returns true if the value is found, and false otherwise.

Example:

				
					const arr = [1, 2, 3, 4];
console.log(arr.includes(3)); // true
console.log(arr.includes(5)); // false

Alternatively, you can use the indexOf() method, which returns the index of the first occurrence of the value if found, or -1 if not found.
console.log(arr.indexOf(3) !== -1); // true
console.log(arr.indexOf(5) !== -1); // false

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

var:
- Function-scoped or globally scoped if declared outside a function.
- Subject to hoisting (variables are moved to the top of their scope).
- Can be redeclared and reassigned.
let:
- Block-scoped (limited to the nearest enclosing block).
- Not subject to hoisting in the same way as var (temporal dead zone applies).
- Can be reassigned but not redeclared.
const:
- Block-scoped.
- Not subject to hoisting (temporal dead zone applies).
- Cannot be reassigned or redeclared.
- Must be initialized at declaration.

Example:

				
					var x = 1;
var x = 2; // Allowed
let y = 3;
// let y = 4; // Error: Identifier 'y' has already been declared
const z = 5;
// z = 6; // Error: Assignment to constant variable.

12. Explain the concept of scope in JavaScript.

Scope refers to the region of code where a variable is accessible. JavaScript has two main types of scope:

Global Scope: Variables declared outside any function or block are in the global scope and can be accessed from anywhere in the code.
Local Scope: Variables declared inside a function or block are in the local scope and are only accessible within that function or block.

JavaScript also has block scope with let and const, which limits variables to the nearest enclosing block (e.g., inside if statements or loops).

Example:

				
					let globalVar = "I'm global";
function outer() {
    let outerVar = "I'm in outer";
    if (true) {
        let blockVar = "I'm in block";
        console.log(globalVar); // Accessible
        console.log(outerVar); // Accessible
        console.log(blockVar); // Accessible
    }
    // console.log(blockVar); // Error: blockVar is not defined
}
// console.log(outerVar); // Error: outerVar is not defined

13. What are arrow functions in JavaScript, and how do they differ from regular functions?

Arrow functions are a shorthand syntax for writing functions in JavaScript. They were introduced in ES6 and are particularly useful for concise function expressions.

Key Differences from Regular Functions:

Feature	Arrow Functions	Regular Functions
Syntax	(a, b) => a + b	function(a, b) { return a + b; }
this Binding	Lexical (inherits from surrounding scope)	Dynamic (depends on how called)
arguments Object	Not available	Available
Concise Body	Implicit return for single expression	Explicit return required

Example:

				
					// Regular function
const add = function(a, b) {
    return a + b;
};

// Arrow function
const addArrow = (a, b) => a + b;

14. What is the ‘this’ keyword in JavaScript, and how does it work?

The this keyword refers to the current execution context of a function. Its value depends on how the function is called:

Global Context: In the global scope, this refers to the global object (e.g., window in browsers).
Function Context: Inside a regular function, this depends on how the function is called:
- As a method of an object: this refers to the object.
- As a standalone function: this refers to the global object (or undefined in strict mode).
- With new: this refers to the newly created object.
Arrow Functions: this is lexically bound, meaning it inherits this from the surrounding scope.

Example:

				
					const obj = {
    name: "Alice",
    greet: function() {
        console.log(`Hello, ${this.name}`);
    }
};

obj.greet(); // "Hello, Alice"

const greet = obj.greet;
greet(); // "Hello, undefined" (in non-strict mode)

15. Explain the difference between synchronous and asynchronous code in JavaScript.

Synchronous Code: Executes line by line, blocking further execution until each line is complete. It is easier to understand but can lead to performance issues if long-running tasks are involved.
Asynchronous Code: Allows other code to run while waiting for a task (like a network request) to complete. It uses mechanisms like callbacks, promises, and async/await to handle non-blocking operations.

Example:

				
					// Synchronous
console.log("Start");
for (let i = 0; i < 5; i++) {
    console.log(i);
}
console.log("End");

// Asynchronous
console.log("Start");
setTimeout(() => {
    console.log("This is async");
}, 1000);
console.log("End");

Output:
Start
0
1
2
3
4
End
This is async

16. What is the Event Loop in JavaScript?

The Event Loop is a mechanism that enables JavaScript to handle asynchronous operations without blocking the main thread. It continuously checks for tasks in the event queue and executes them when the call stack is empty.

Key Components:

Component	Description
Call Stack	Where synchronous code is executed.
Event Queue	Where asynchronous tasks are queued.
Event Loop	Monitors the call stack and event queue, executing queued tasks when the stack is clear.

Example:

				
					console.log("Start");
setTimeout(() => console.log("Timeout"), 0);
console.log("End");

Output:
Start
End
Timeout

Here, setTimeout schedules a task, but it doesn’t block the main thread. The Event Loop ensures that “Timeout” is logged after “End”.

17. How does JavaScript handle errors, and what is the try-catch block?

JavaScript handles errors using try-catch blocks. The try block contains code that might throw an error, and the catch block handles the error if it occurs.

Example:

				
					try {
    console.log(x); // ReferenceError: x is not defined
} catch (error) {
    console.error("An error occurred:", error);
}

Output:
An error occurred: ReferenceError: x is not defined

You can also use finally to execute code regardless of whether an error occurred.

18. What are modules in JavaScript, and how do you use them?

Modules allow you to organize code into separate files and reuse functionality across different parts of your application. They were introduced in ES6.

Key Features:

Export: Use export to make variables, functions, or classes available to other modules.
Import: Use import to bring in exported items from other modules.

Example (math.js):

				
					export function add(a, b) {
    return a + b;
}

In another file (app.js):
import { add } from './math.js';
console.log(add(2, 3)); // 5

Modules help keep code modular and maintainable. To learn more about modules, explore our master DSA, web dev, and system design course.

19. Explain the concept of prototypal inheritance in JavaScript.

JavaScript uses prototypal inheritance, where objects inherit properties and methods from other objects (prototypes). Every object has a __proto__ property that links to its prototype.

Example:

				
					const animal = {
    eat() {
        console.log("Eating...");
    }
};

const dog = {
    __proto__: animal,
    bark() {
        console.log("Woof!");
    }
};

dog.eat(); // "Eating..."
dog.bark(); // "Woof!"

Here, dog inherits the eat method from animal through its prototype chain.

20. What are some best practices for writing clean and maintainable JavaScript code?

Use meaningful variable names: Avoid single-letter variables; use descriptive names.
Follow consistent naming conventions: Use camelCase for variables and functions.
Write modular code: Break code into smaller functions or modules for better readability.
Use strict mode: Enable strict mode (‘use strict’) to catch common errors.
Avoid global variables: Use modules or closures to encapsulate variables.
Handle errors gracefully: Use try-catch for error handling.
Write comments: Add comments to explain complex logic.
Test your code: Use testing frameworks like Jest or Mocha to ensure reliability.

For more coding best practices, consider our crash course.

Conclusion

Mastering JavaScript is crucial for any web developer in 2025. The questions covered here span basic to advanced concepts and are likely to be asked in interviews. To deepen your understanding of JavaScript, consider taking our web development course. Additionally, mastering data structures and algorithms is crucial for any developer, so check out our DSA course. For a comprehensive learning path, explore our master DSA, web dev, and system design course or our data science course for related skills. Remember, practice is key—build projects, solve coding challenges, and review these questions regularly to ensure you’re prepared for your next interview.

FAQs

How can I prepare for a JavaScript interview?

To prepare for a JavaScript interview, start by understanding the basics of the language, including data types, functions, and scope. Then, move on to more advanced topics like closures, promises, and asynchronous programming. Practice coding problems on platforms like LeetCode or HackerRank, and review common interview questions from resources like Toptal or GeeksforGeeks. Additionally, building small projects can help solidify your knowledge.

What are some popular JavaScript frameworks I should know?

Some popular JavaScript frameworks include React for building user interfaces, Angular for developing single-page applications, and Vue.js for creating interactive web interfaces. Understanding at least one of these frameworks can be beneficial, as many companies use them in their tech stacks.

Is it necessary to know TypeScript for JavaScript interviews?

While not always necessary, knowing TypeScript can be advantageous, especially for roles that involve working with large codebases or teams that use TypeScript. TypeScript adds static typing to JavaScript, which can help catch errors early and improve code maintainability.

How important is understanding the DOM for JavaScript development?

Understanding the Document Object Model (DOM) is crucial for frontend JavaScript development. The DOM represents the structure of an HTML document and allows you to manipulate it using JavaScript. Knowing how to select elements, add event listeners, and modify the DOM is essential for creating interactive web pages.

What is the best way to learn JavaScript?

The best way to learn JavaScript is through a combination of reading documentation, taking online courses, practicing coding exercises, and building projects. Resources like MDN Web Docs, freeCodeCamp, and Udemy courses can be very helpful. Additionally, contributing to open-source projects or participating in coding challenges can enhance your learning experience.

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Top 20 JavaScript Interview Questions and Answers for 2025

Introduction

Top 20 JavaScript Interview Questions and Answers

1. What is JavaScript, and what are its main features?

2. What is the difference between JavaScript and Java?

3. What are the different data types in JavaScript?

4. Explain the concept of hoisting in JavaScript.

5. What is the difference between null and undefined in JavaScript?

6. What are closures in JavaScript?

7. Explain callback functions in JavaScript.

8. What are promises in JavaScript, and how do they work?

10. How can you check if an array includes a certain value in JavaScript?

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

12. Explain the concept of scope in JavaScript.

13. What are arrow functions in JavaScript, and how do they differ from regular functions?

14. What is the ‘this’ keyword in JavaScript, and how does it work?

15. Explain the difference between synchronous and asynchronous code in JavaScript.

16. What is the Event Loop in JavaScript?

17. How does JavaScript handle errors, and what is the try-catch block?

18. What are modules in JavaScript, and how do you use them?

20. What are some best practices for writing clean and maintainable JavaScript code?

Conclusion

FAQs

How can I prepare for a JavaScript interview?

What are some popular JavaScript frameworks I should know?

Is it necessary to know TypeScript for JavaScript interviews?

How important is understanding the DOM for JavaScript development?

What is the best way to learn JavaScript?

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