JavaScript Interview Cheatsheet: Questions Every Developer Must Know

JavaScript is one of the most popular programming languages in the world, powering everything from simple websites to complex web applications. If you are preparing for a job interview as a front-end or full-stack developer, being confident in JavaScript is a must. To help you succeed, this guide covers essential JavaScript interview questions and Answers with examples.

1. What is JavaScript?

Answer:
JavaScript is a lightweight, interpreted programming language primarily used to make web pages interactive. It runs on the client side in browsers but can also run on the server using environments like Node.js. JavaScript is single-threaded, dynamically typed, and supports event-driven, functional, and object-oriented programming styles.

2. Difference between var, let, and const?

Answer:

• var: Function-scoped, can be redeclared, hoisted with value undefined.

• let: Block-scoped, cannot be redeclared in the same block, hoisted but not initialized.

• const: Block-scoped, must be initialized, cannot be reassigned.

Example:

var x = 10;
let y = 20;
const z = 30;

3. What are closures?

Answer:
A closure is a function that retains access to its outer scope even after the outer function has executed.

Example:

function outer() {
  let count = 0;
  return function inner() {
    count++;
    return count;
  };
}
const counter = outer();
console.log(counter()); // 1
console.log(counter()); // 2

Closures are widely used for data hiding and creating private variables.

4. Explain == vs. ===.

Answer:

• ==: Compares values after type conversion (loose equality).

• ===: Compares both value and type (strict equality).

Example:

5 == "5";   // true
5 === "5";  // false

5. What is hoisting?

Answer:
Hoisting is JavaScript’s behavior of moving declarations to the top of the current scope.

• Function declarations are fully hoisted.

• Variables declared with var are hoisted but initialized as undefined.

• let and const are hoisted but remain in the temporal dead zone until initialized.

6. What are arrow functions?

Answer:
Arrow functions are a shorter syntax for writing functions introduced in ES6. They do not have their own this, making them useful in callbacks.

Example:

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

7. What is the difference between synchronous and asynchronous programming?

Answer:

• Synchronous: Code runs line by line; next task waits for the current one to complete.

• Asynchronous: Tasks don’t block execution; callbacks, promises, or async/await handle results later.

Example:

console.log("Start");
setTimeout(() => console.log("Async Task"), 1000);
console.log("End");

Output:

Start
End
Async Task

8. Explain Promises and async/await.

Answer:

• A Promise represents the eventual completion or failure of an asynchronous operation.

• async/await makes asynchronous code look synchronous and easier to read.

Example:

function fetchData() {
  return new Promise(resolve => {
    setTimeout(() => resolve("Data received"), 1000);
  });
}

async function getData() {
  const result = await fetchData();
  console.log(result);
}
getData();

9. What is event bubbling and capturing?

Answer:

• Event bubbling: Event propagates from the innermost element to the outermost.

• Event capturing: Event propagates from the outermost element to the innermost.
  JavaScript event listeners use bubbling by default, but you can enable capturing with a third parameter true in addEventListener.

10. What are JavaScript data types?

Answer:

• Primitive types: String, Number, Boolean, Null, Undefined, Symbol, BigInt

• Non-primitive: Objects, Arrays, Functions

11. Difference between null and undefined?

Answer:

• undefined: A variable declared but not assigned a value.

• null: An assignment value representing “no value.”

12. What is the difference between call(), apply(), and bind()?

Answer:

• call(): Invokes a function with arguments passed individually.

• apply(): Invokes a function with arguments passed as an array.

• bind(): Returns a new function with this bound to the provided value.

Example:

function greet(msg) {
  console.log(msg + " " + this.name);
}
const person = { name: "Suraj" };
greet.call(person, "Hello");
greet.apply(person, ["Hi"]);
const bound = greet.bind(person, "Hey");
bound();

13. What are higher-order functions?

Answer:
Functions that take other functions as arguments or return a function. Examples: map(), filter(), reduce().

Example:

const numbers = [1, 2, 3];
const doubled = numbers.map(n => n * 2);

14. Explain the difference between localStorage, sessionStorage, and cookies.

• localStorage: Stores data with no expiration, per origin.

• sessionStorage: Stores data for one session, cleared on tab close.

• cookies: Small pieces of data sent with every HTTP request; used for authentication.

15. What is the event loop?

Answer:
The event loop allows JavaScript (single-threaded) to handle asynchronous tasks.

• Call Stack: Executes code line by line.

• Web APIs: Handle async tasks like setTimeout.

• Callback Queue: Stores callbacks.

• Event Loop: Pushes callbacks to the call stack when it’s free.

Final Thoughts

JavaScript interviews and Answers often test both theoretical understanding and problem-solving skills. By practicing these core questions—covering variables, functions, closures, promises, event handling, and storage—you’ll build confidence to tackle both beginner and advanced interview rounds.

Remember, beyond memorizing answers, the key is to practice coding examples so you can explain concepts clearly and demonstrate them live in interviews. Join Us Tpoint Tech.