90+ Exclusive JavaScript Interview Questions and Answers to Know [2025]

Here are the most common JavaScript interview questions for experienced web developers:

1. What is function binding in JavaScript and how do you use it?

JavaScript enables developers to determine ‘this’ keyword references through explicit value assignment in functions. The .bind(), .call(), and .apply() are common methods for accomplishing this task.

The usage of binding becomes essential when working with callbacks and event handlers, which could potentially break ‘this’ reference.

Example:

const user = {

  name: “John”,

};

function greet() {

  console.log(`Hello, ${this.name}`);

}

constgreetUser = greet.bind(user);

greetUser(); // Output: Hello, John

The usage of binding becomes essential when working with callbacks together with event handlers that could potentially break ‘this’ reference.

2. How does JavaScript’s inheritance model work in ES6?

ES6 implements class-based inheritance, yet its operational model operates through prototypal inheritance under the surface. Classes can extend other classes using the ‘extends’ keyword.

Example:

class Animal {

  constructor(name) {

    this.name = name;

  }

}

class Dog extends Animal {

  bark() {

    console.log(`${this.name} barks!`);

  }

}

const dog = new Dog(‘Tommy’);

dog.bark(); // Output: Tommy barks!

Using prototypal inheritance allows programmers to create structured and easy-to-understand class inheritance frameworks in contemporary JavaScript.

3. What is JSX and how does it relate to ReactJS?

JSX (JavaScript XML) operates as a JavaScript format extension used exclusively in ReactJS development. Developers can create HTML similar code directly inside their JavaScript programs through JSX.

Example:

const element = <h1>Hello, World!</h1>;

Indexed JSX improves both the clarity of code and enables dynamic value integration throughout the user interface components.

4. How does JavaScript’s ‘this’ keyword work in object methods?

The ‘this’ keyword inside an object method represents the object which invokes the method while running the code.

Example:

const person = {

  name: “Alice”,

  greet() {

    console.log(`Hi, I am ${this.name}`);

  },

};

person.greet(); // Output: Hi, I am Alice

The value of ‘this’ inside event handlers and callbacks should be handled properly because it changes dynamically.

5. What are Promises, and how are they used in modern JavaScript frameworks?

In JavaScript programming, promises act as representations of both successful and failed termination scenarios of asynchronous operations. Promises reduce the complexity of asynchronous code processing and prevent asynchronous function nesting issues.

Example:

constfetchData = () => {

  return new Promise((resolve, reject) => {

    setTimeout(() => resolve(“Data loaded”), 1000);

  });

};

fetchData().then(data => console.log(data));

Frameworks like ReactJS and Angular use Promises extensively for API calls and async processes.

6. What is localStorage and sessionStorage in JavaScript, and how are they different?

The JavaScript Web Storage API includes both localStorage and sessionStorage features as its members.

Example:

localStorage.setItem(‘name’, ‘John’);

sessionStorage.setItem(‘role’, ‘Admin’);

7. What is throttling and how does it differ from debouncing in JavaScript?

Within JavaScript frameworks, the execution of functions can only occur once during preset periods. Function execution through debouncing is delayed until there is a consistent time period without event detections.

Throttling Example:

function throttle(func, limit) {

  letinThrottle;

  return function() {

    if (!inThrottle) {

      func();

      inThrottle = true;

      setTimeout(() => (inThrottle = false), limit);

    }

  };

}

8. How would you implement pagination in JavaScript?

Using JavaScript pagination helps to distribute content across separate pages.

Example:

function paginate(array, pageSize, pageNumber) {

  returnarray.slice((pageNumber – 1) * pageSize, pageNumber * pageSize);

}

const data = [1,2,3,4,5,6,7,8,9,10];

console.log(paginate(data, 3, 2)); // Output: [4,5,6]

9. What is the purpose of async/await in JavaScript and how does it work?

Writing asynchronous code becomes synchronous through the implementation of async/await in JavaScript.

Example:

async function fetchData() {

  const response = await fetch(‘https://api.example.com/data’);

  const data = await response.json();

  console.log(data);

}

The programming technique enhances both the clarity of code and error management processes.

10. How would you handle caching in JavaScript to improve performance?

The implementation of JavaScript caching allows data storage in memory and localStorage to enhance speed when accessing data.

Example:

const cache = {};

functionfetchData(id) {

  if (cache[id]) {

    returnPromise.resolve(cache[id]);

  }

  return fetch(`/api/data/${id}`)

    .then(response =>response.json())

    .then(data => {

      cache[id] = data;

      return data;

    });

}

11. What are JavaScript modules and how are they used in large-scale applications?

JavaScript provides modules to create reusable code sections. JavaScript modules allow files to exchange code through import and export functions.

Example:

// module.js

exportconst name = ‘John’;

// app.js

import { name } from ‘./module.js’;

They promote better code organization.

12. How do you handle state management in JavaScript applications?

JavaScript applications can manage state through built-in options including_reaction’suseState or Redux and Context API_.

Example using React’suseState:

const [count, setCount] = useState(0);

13. How does ReactJS integrate with JavaScript to create dynamic UIs?

ReactJS enables developers to construct components using JavaScript as a framework through which they control local state and trigger automatic rendering according to state changes.

Example:

function Counter() {

  const [count, setCount] = useState(0);

  return<button onClick={() =>setCount(count + 1)}>{count}</button>;

}

14. How do you debug JavaScript code efficiently?

Efficient debugging in JavaScript involves using:

• console.log()
• Chrome DevTools
• Breakpoints
• Debugger statement
• Linting tools like ESLint

Example:

debugger; // Pauses execution at this line

15. What is memoization and how does it enhance performance in JavaScript?

In JavaScript, memoization creates a cache of function results according to the arguments that were used when the function was called.

Example:

functionmemoize(fn) {

  const cache = {};

  return function(…args) {

    const key = JSON.stringify(args);

    if (!cache[key]) {

      cache[key] = fn(…args);

    }

    return cache[key];

  };

}

const add = (a, b) => a + b;

constmemoizedAdd = memoize(add);

The method decreases superfluous computational processes while improving operational efficiency.