50 JavaScript Interview Questions and Answers (2026)

JavaScript Interview Questions (2026)

Top 50 JavaScript interview questions with clear answers. Bookmark this for your next interview.

1. What is the difference between let, const, and var?

var is function-scoped and hoisted. let is block-scoped and not accessible before declaration. const is block-scoped and cannot be reassigned after initialization.

2. What is a closure?

A closure is a function that has access to variables from its outer (enclosing) function scope, even after the outer function has returned. Closures are created every time a function is created.

3. What is the event loop?

JavaScript is single-threaded. The event loop continuously checks the call stack and the callback queue. When the call stack is empty, it picks the first callback from the queue and pushes it to the call stack.

4. What is the difference between == and ===?

== performs type coercion before comparison. === checks both value and type without coercion. Always prefer === for predictable comparisons.

5. What are Promises?

A Promise represents the eventual completion (or failure) of an asynchronous operation. It has three states: pending, fulfilled, and rejected. Use async/await for cleaner async code.

6. What is destructuring?

const { name, age } = person;
const [first, second] = array;
const { name: userName = 'default' } = person;

7. What is the spread operator?

const arr2 = [...arr1, 4, 5]; // Copy and extend array
const obj2 = { ...obj1, key: 'value' }; // Copy and extend object

8. What is a higher-order function?

A function that takes another function as an argument or returns a function. Examples: map, filter, reduce, forEach.

9. What is async/await?

async function fetchData() {
  try {
    const response = await fetch('/api/data');
    const data = await response.json();
    return data;
  } catch (error) {
    console.error(error);
  }
}

10. What is the difference between null and undefined?

undefined means a variable is declared but not assigned. null is an intentional absence of value. typeof undefined is 'undefined'. typeof null is 'object' (a known JS bug).

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