Top 20 JavaScript Interview Questions with Answers for Front-End Developers
If you're preparing for a Front-End Developer interview, you can expect a range of JavaScript questions that test your knowledge of both core concepts and advanced techniques. Below are 20 commonly asked JavaScript interview questions along with detailed answers to help you prepare.
1. What is JavaScript?
Answer:
JavaScript is a dynamic programming language that is primarily used for client-side web development. It enables interactive web pages by allowing you to implement complex features like event handling, DOM manipulation, and asynchronous operations. It is also used in server-side programming through environments like Node.js.
2. What are the different data types in JavaScript?
Answer:
JavaScript has primitive and non-primitive data types:
- Primitive:
String, Number, Boolean, Undefined, Null, Symbol, BigInt
- Non-primitive (Objects):
Object, Array, Function
3. What is the difference between let, var, and const?
Answer:
var: Function-scoped and can be re-declared and updated.let: Block-scoped, cannot be re-declared but can be updated.const: Block-scoped, cannot be re-declared or updated.
4. Explain the concept of Hoisting in JavaScript.
Answer:
Hoisting is JavaScript’s default behavior of moving declarations to the top of the current scope. Variables and function declarations are hoisted, but variable assignments are not.
Example:
js
console.log(x); var x = 5;
5. What are Closures in JavaScript?
Answer:
A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope.
Example:
js
function outer() { let name = "JavaScript"; return function inner() { console.log(name); };}
6. What is the difference between == and === in JavaScript?
Answer:
==: Checks for equality after performing type conversion (loose equality).===: Checks for equality without performing type conversion (strict equality).
Example:
7. What is the purpose of the this keyword in JavaScript?
Answer:
The this keyword refers to the context in which a function is executed. Its value depends on how a function is called:
- In a method,
this refers to the object.
- In a regular function,
this refers to the global object (window in browsers).
- In arrow functions,
this is lexically scoped.
8. What is an IIFE (Immediately Invoked Function Expression)?
Answer:
An IIFE is a function that is executed immediately after it is defined. It is commonly used to create a new scope and avoid polluting the global namespace.
Example:
js
(function() { console.log("IIFE executed");})();
9. Explain Event Delegation in JavaScript.
Answer:
Event Delegation allows you to attach a single event listener to a parent element to manage events on multiple child elements. It leverages event bubbling, where an event propagates up from the target element to its ancestors.
10. What is the difference between call(), apply(), and bind()?
Answer:
call(): Invokes a function with a specified this context and individual arguments.apply(): Similar to call(), but arguments are passed as an array.bind(): Returns a new function with a specified this context but does not execute it immediately.
11. What is the event loop in JavaScript?
Answer:
The event loop allows JavaScript to perform asynchronous operations. It handles execution of the call stack and the callback queue. Tasks in the call stack execute first, and once it's empty, tasks from the callback queue are moved to the call stack.
12. What are Promises in JavaScript?
Answer:
A Promise is an object representing the eventual completion or failure of an asynchronous operation. It has three states:
Pending: Initial state.Fulfilled: Operation completed successfully.Rejected: Operation failed.
Example:
js
let promise = new Promise((resolve, reject) => { if (success) resolve("Success"); else reject("Error");});
13. What is the difference between synchronous and asynchronous code?
Answer:
- Synchronous code: Executes line-by-line, where each operation must complete before the next one starts.
- Asynchronous code: Executes without blocking further operations, using callbacks, promises, or async/await for handling results when they become available.
14. What is null and undefined in JavaScript?
Answer:
- null: Represents the intentional absence of any object value.
- undefined: Represents a variable that has been declared but not yet assigned a value.
15. What is a JavaScript Prototype?
Answer:
Every JavaScript object has a prototype, which is another object from which it inherits properties and methods. In JavaScript, inheritance is prototype-based rather than class-based.
16. What is the new keyword in JavaScript?
Answer:
The new keyword is used to create an instance of an object from a constructor function. It does four things:
- Creates a new empty object.
- Sets the
this context to the new object.
- Links the new object to a prototype.
- Returns the new object.
17. What are Arrow Functions, and how are they different from regular functions?
Answer:
Arrow functions provide a shorter syntax and have lexical scoping for the this keyword. Unlike regular functions, the value of this in arrow functions is inherited from the outer scope.
18. What is NaN in JavaScript?
Answer:
NaN stands for "Not-a-Number." It is a value that results from an invalid number operation (e.g., dividing a string by a number). You can check if a value is NaN using the isNaN() function.
19. What is Debouncing in JavaScript?
Answer:
Debouncing is a technique to limit the rate at which a function is executed. If a function is called frequently, debouncing ensures that it is only executed after a specific amount of time has passed since the last call.
Example:
js
function debounce(func, delay) { let timeout; return function() { clearTimeout(timeout); timeout = setTimeout(() => func.apply(this, arguments), delay); };}
20. What are JavaScript modules?
Answer:
JavaScript modules allow you to break up your code into separate files, making it easier to manage. Using export and import statements, you can share code between different files.
Example:
js
export function sayHello() { console.log("Hello");}import { sayHello } from './module.js';sayHello();
These questions and answers cover essential concepts in JavaScript and are frequently asked in front-end developer interviews. Understanding them thoroughly will help you excel in your interview preparation.
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