JavaScript Interview Questions

Top JavaScript interview questions — from junior to senior level.

Variables & Types

• What's the difference between var, let, and const?
• What are the primitive types in JavaScript?
• What is type coercion? Give examples.
• What are falsy values?
• What is NaN? How do you check for it?

Scope & Closures

• What is scope? Types of scope in JS?
• What is hoisting?
• What is a closure? Give a practical example.
• What is the Temporal Dead Zone?

Functions

• Difference between function declaration and expression?
• What are higher-order functions?
• What is currying?
• What is an IIFE?
• What is the difference between call, apply, and bind?

OOP & Prototypes

• How does prototypal inheritance work?
• What is this? How does it change context?
• Difference between class and prototype-based inheritance?

Async JavaScript

• How does the event loop work?
• What are microtasks vs macrotasks?
• What is a Promise? States?
• Difference between async/await and Promises?
• What is Promise.all vs Promise.allSettled?
• What are generators?

Modern JavaScript

• What is destructuring?
• Difference between rest and spread?
• What are WeakMap and WeakSet?
• What is the difference between == and ===?
• How does Symbol work?

Performance & Memory

• How does garbage collection work in JavaScript?
• What is the V8 engine?
• What is a memory leak? How to prevent it?

Security

• What is XSS? How to prevent it?
• What is CORS? How does it work?
• What is CSRF?

Sample Answers

What is a closure?

// A closure is a function that remembers variables from
// its outer scope even after the outer function has returned.

function makeAdder(x) {
  return function(y) {
    return x + y  // 'x' is closed over
  }
}

const add5 = makeAdder(5)
add5(3)  // 8
add5(10) // 15

What is the event loop?

JavaScript is single-threaded. The event loop allows non-blocking operations by offloading work to Web APIs, then queuing callbacks. Microtasks (Promises) are processed before macrotasks (setTimeout).