Top JavaScript Interview Questions for 5

JavaScript plays a fundamental role in web development and is an essential skill for both front-end and full-stack developers. Whether you are an experienced professional or gearing up for your first interview, mastering key JavaScript concepts is vital. Here’s a handpicked collection of the top 20 JavaScript interview questions to help you excel in your next interview.

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1. What is JavaScript, and how does it differ from Java?

JavaScript is a lightweight, interpreted programming language primarily used for web development. It is object-oriented but differs from Java in syntax, purpose, and execution environment. JavaScript is dynamically typed and runs in browsers, while Java is statically typed and requires the JVM.

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2. What are the different data types in JavaScript?

JavaScript has seven primitive data types:

• String
• Number
• Boolean
• Undefined
• Null
• BigInt
• Symbol

Additionally, non-primitive data types include objects, arrays, and functions.

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3. Explain “hoisting” in JavaScript.

Hoisting is JavaScript's default behavior of moving declarations to the top of the scope. For example:

console.log(a); // undefined
var a = 5;

In this example, var a is hoisted but not its initialization.

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

• var: Function-scoped, can be re-declared, and may lead to bugs due to hoisting.
• let: Block-scoped and does not allow re-declaration.
• const: Block-scoped and used for constants; cannot be reassigned.

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5. What are closures in JavaScript?

A closure is a function that remembers its outer scope even after the scope has exited.

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

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

• ==: Compares values after type coercion.
• ===: Compares both value and type.

Example:

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

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7. What are JavaScript Promises?

Promises represent asynchronous operations. They can be in one of three states:

• Pending
• Resolved
• Rejected

Example:

console.log(a); // undefined
var a = 5;

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8. Explain the concept of async/await.

async/await simplifies working with Promises, making asynchronous code appear synchronous.

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

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9. What is event delegation in JavaScript?

Event delegation uses a parent element to handle events for its child elements. It improves performance by reducing event listeners.

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

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10. What are JavaScript modules?

Modules allow you to organize code into separate files, making it reusable and maintainable.

Example:

• Export:

const promise = new Promise((resolve, reject) => {
  resolve("Success");
});
promise.then(data => console.log(data));

• Import:

async function fetchData() {
  const response = await fetch('https://api.example.com/data');
  const data = await response.json();
  console.log(data);
}

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11. Explain the difference between call(), apply(), and bind().

• call(): Invokes a function with a specified this value and arguments.
• apply(): Similar to call(), but takes arguments as an array.
• bind(): Returns a new function with a specified this value.

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12. What is the difference between synchronous and asynchronous JavaScript?

• Synchronous: Executes line by line.
• Asynchronous: Executes non-blocking tasks, allowing the program to continue running.

Example of asynchronous:

document.getElementById('parent').addEventListener('click', (e) => {
  if (e.target && e.target.tagName === 'BUTTON') {
    console.log('Button clicked!');
  }
});

Output:

World (first)

Hello (second)

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13. What is the DOM in JavaScript?

The Document Object Model (DOM) is a tree structure representing a web page. JavaScript interacts with the DOM to manipulate elements dynamically.

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14. What are higher-order functions in JavaScript?

Higher-order functions take other functions as arguments or return them as results.

Example:

export const greet = () => console.log("Hello");

Fold Function in JavaScript

15. What is the difference between null and undefined?

• null: Explicitly represents no value.
• undefined: Represents an uninitialized value.

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16. What is memoization in JavaScript?

Memoization is an optimization technique to cache function results.

Example:

import { greet } from './module.js';

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17. What is the event loop in JavaScript?

The event loop handles asynchronous operations, ensuring non-blocking execution.

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18. What are JavaScript prototypes?

Every object in JavaScript has a prototype. Methods and properties shared among instances are defined on the prototype.

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19. What is the difference between for...of and for...in loops?

• for...of: Iterates over iterable objects like arrays.
• for...in: Iterates over enumerable properties of an object.

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20. How does JavaScript handle errors?

Errors are handled using try...catch.

Example:

console.log(a); // undefined
var a = 5;

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These questions provide a strong foundation for acing JavaScript interviews. Make sure to practice coding examples and understand the underlying concepts to confidently demonstrate your skills. For more web development tips, visit Makemychance.

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