Top 60+ JavaScript Interview Questions and Answers (2024)

📢Introduction

JavaScript is a fundamental programming language for web development. Whether you’re creating a simple website or a complex web application, JavaScript is essential. Preparing for a JavaScript interview can be challenging, but with the right questions and answers, you can confidently showcase your knowledge. This blog will guide you through the top 60+ JavaScript interview questions and answers to help you excel in your 2024 interviews.

Basic JavaScript Questions📝

1. What is JavaScript?

JavaScript is a programming language that allows you to create dynamic content on websites. It’s responsible for things like interactive forms, animations, and real-time updates.

2. What are var, let, and const?

• var: Declares a variable with function scope, meaning it’s accessible throughout the function.
• let: Declares a variable with block scope, meaning it’s only accessible within the block it’s declared in.
• const: Declares a constant variable, which cannot be reassigned after its initial assignment.

3. What are JavaScript data types?

JavaScript has several data types, including:

• String: Text, like "Hello, World!".
• Number: Numbers, like 5 or 3.14.
• Boolean: True or false values.
• Object: Collections of key-value pairs.
• Undefined: A variable that has been declared but not assigned a value.
• Null: Represents the intentional absence of any object value.

4. What are arrays in JavaScript?

Arrays are collections of values. You can store multiple values in an array and access them by their index, such as let fruits = ["apple", "banana", "cherry"];.

5. What is a function in JavaScript?

A function is a block of code designed to perform a specific task. It can take inputs, called parameters, and return a result. For example:

function add(a, b) {
  return a + b;
}

Intermediate JavaScript Questions📖

6. What is this in JavaScript?

The this keyword refers to the object that is currently executing the code. Its value changes depending on where it’s used—inside a function, it usually refers to the object that owns the function.

7. What is prototypal inheritance?

Prototypal inheritance is a feature in JavaScript where objects can inherit properties and methods from another object, called the prototype. This allows for shared behavior across objects without duplicating code.

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

• ==: Compares two values for equality after converting them to a common type (loose equality).
• ===: Compares two values for equality without converting their types (strict equality).

9. What are arrow functions?

Arrow functions are a shorthand way to write functions in JavaScript. They are more concise and do not have their own this value. Example:

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

10. What is a higher-order function?

A higher-order function is a function that takes another function as an argument, returns a function, or both. Examples include map, filter, and reduce.

Advanced JavaScript Questions🚀

11. What is event delegation?

Event delegation is a technique where you add a single event listener to a parent element to handle events for multiple child elements. This reduces the number of event listeners in your code and improves performance.

12. What are async and await?

async and await are keywords used to handle asynchronous operations more easily. async marks a function as asynchronous, and await pauses the function execution until a promise is resolved.

13. What is a JavaScript Promise?

A promise is an object representing the eventual completion or failure of an asynchronous operation. Promises make it easier to work with asynchronous code by using .then() for success and .catch() for failure.

14. What is hoisting in JavaScript?

Hoisting is JavaScript’s behavior of moving variable and function declarations to the top of their containing scope before code execution. This means you can use functions and variables before they are declared in your code.

15. What is the event loop?

The event loop is a mechanism that allows JavaScript to perform non-blocking operations by offloading tasks like I/O operations to the system and handling them when they’re ready, even while other code is still running.

16. What is closure in JavaScript?

A closure is a feature in JavaScript where an inner function has access to variables from its outer function’s scope, even after the outer function has returned. Closures are useful for creating private variables and functions.

Example:

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

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

• call(): Invokes a function with a specified this value and arguments passed individually.
• apply(): Similar to call(), but arguments are passed as an array.
• bind(): Returns a new function with a specified this value and arguments, allowing you to set this and partially apply arguments.

Example:

function greet(greeting, name) {
  console.log(greeting + ', ' + name);
}
greet.call(null, 'Hello', 'Alice');  // "Hello, Alice"
greet.apply(null, ['Hi', 'Bob']);    // "Hi, Bob"
const greetHello = greet.bind(null, 'Hello');
greetHello('Charlie');               // "Hello, Charlie"

18. What is the event.preventDefault() method?

The event.preventDefault() method stops the default action of an element from occurring, such as preventing a form from submitting or stopping a link from navigating to a URL.

Example:

document.getElementById('myForm').addEventListener('submit', function(event) {
  event.preventDefault();
  alert('Form submission canceled!');
});

19. What is the difference between null and undefined?

• undefined: A variable that has been declared but not assigned a value.
• null: An intentional absence of any object value. It’s an assignment value that represents no value.

Example:

let a;
console.log(a);  // undefined
let b = null;
console.log(b);  // null

20. What are template literals in JavaScript?

Template literals are a way to include expressions and variables within a string, using backticks ` instead of quotes. They allow for multi-line strings and string interpolation.

Example:

const name = 'John';
const message = `Hello, ${name}!`;
console.log(message); // "Hello, John!"

21. What is the purpose of the map() method in JavaScript?

The map() method creates a new array by calling a provided function on every element in the original array. It’s used to transform data in an array without mutating the original array.

Example:

const numbers = [1, 2, 3];
const doubled = numbers.map(num => num * 2);
console.log(doubled); // [2, 4, 6]

22. What is the reduce() method, and how does it work?

The reduce() method executes a reducer function on each element of the array, resulting in a single output value. It’s useful for summing values, combining arrays, or any other cumulative operation.

Example:

const numbers = [1, 2, 3, 4];
const sum = numbers.reduce((accumulator, currentValue) => accumulator + currentValue, 0);
console.log(sum); // 10

23. What are arrow functions, and how do they differ from regular functions?

Arrow functions provide a shorter syntax for writing functions and do not have their own this, arguments, or new.target. They also cannot be used as constructors.

Example:

const add = (a, b) => a + b;
console.log(add(2, 3)); // 5

24. What is destructuring in JavaScript?

Destructuring is a syntax that allows you to unpack values from arrays or properties from objects into distinct variables.

Example:

const [a, b] = [1, 2];
console.log(a); // 1
console.log(b); // 2

const {name, age} = {name: 'Alice', age: 25};
console.log(name); // Alice
console.log(age); // 25

25. What is the spread operator?

The spread operator (...) allows an iterable (like an array) to be expanded in places where multiple elements or arguments are expected.

Example:

const arr1 = [1, 2, 3];
const arr2 = [...arr1, 4, 5, 6];
console.log(arr2); // [1, 2, 3, 4, 5, 6]

26. What are JavaScript Promises and why are they used?

A promise is an object that represents the eventual completion (or failure) of an asynchronous operation. Promises allow you to handle asynchronous operations more gracefully by avoiding callback hell.

Example:

const myPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('Success!');
  }, 2000);
});

myPromise.then(value => {
  console.log(value); // "Success!"
});

27. What is the fetch() API in JavaScript?

The fetch() API provides a way to make network requests similar to XMLHttpRequest. It returns a promise that resolves with the Response object, representing the response to the request.

Example:

fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error('Error:', error));

28. What is a JavaScript module?

A module in JavaScript is a file containing related code that can be imported and exported between other files or modules. This promotes code reusability and organization.

Example:

// math.js
export function add(a, b) {
  return a + b;
}

// main.js
import { add } from './math.js';
console.log(add(2, 3)); // 5

29. What is asynchronous programming in JavaScript?

Asynchronous programming in JavaScript allows for non-blocking code execution. It uses callbacks, promises, or async/await to handle tasks like API calls, file reading, and timers without freezing the application.

Example:

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

30. What is a generator function?

A generator function in JavaScript is a special type of function that can be paused and resumed. It returns a generator object that controls the execution of the function using yield.

Example:

function* generatorFunction() {
  yield 'Hello';
  yield 'World';
}

const generator = generatorFunction();
console.log(generator.next().value); // "Hello"
console.log(generator.next().value); // "World"

31. What is the this keyword in JavaScript?

this refers to the object from which a function was called. Its value is determined by how a function is invoked and can vary based on the context.

Example:

const obj = {
  name: 'Alice',
  greet: function() {
    console.log(this.name);
  }
};
obj.greet(); // "Alice"

32. What are the different ways to create an object in JavaScript?

• Object Literals: { key: value }
• Constructor Functions: function Person(name) { this.name = name; }
• Object.create(): Creates a new object with a specified prototype.
• ES6 Classes: class Person { constructor(name) { this.name = name; } }

33. What is event delegation?

Event delegation is a technique where a single event listener is added to a parent element to manage events for multiple child elements. This is efficient for managing many elements dynamically.

Example:

document.querySelector('#parent').addEventListener('click', function(event) {
  if (event.target.matches('.child')) {
    console.log('Child element clicked:', event.target.textContent);
  }
});

34. What is the typeof operator in JavaScript?

The typeof operator returns a string indicating the type of the operand.

Example:

console.log(typeof 42);          // "number"
console.log(typeof 'Hello');     // "string"
console.log(typeof true);        // "boolean"
console.log(typeof undefined);   // "undefined"
console.log(typeof null);        // "object" (this is a known quirk)

35. What is hoisting in JavaScript?

Hoisting is JavaScript’s behavior of moving variable and function declarations to the top of their containing scope during compilation. This allows functions to be called before they are declared.

Example:

console.log(foo()); // "Hello"
function foo() {
  return 'Hello';
}

36. What is the difference between call(), apply(), and bind() in JavaScript?

• call(): Invokes a function with a given this value and arguments provided one by one.
• apply(): Invokes a function with a given this value, but arguments are provided as an array.
• bind(): Returns a new function with a given this value and optional arguments pre-set.

Example:

function greet(greeting, punctuation) {
  console.log(greeting + ', ' + this.name + punctuation);
}

const person = { name: 'Alice' };

greet.call(person, 'Hello', '!');  // "Hello, Alice!"
greet.apply(person, ['Hi', '.']);  // "Hi, Alice."
const boundGreet = greet.bind(person, 'Hey');
boundGreet('?');  // "Hey, Alice?"

37. What is the for...of loop in JavaScript?

The for...of loop is used to iterate over iterable objects like arrays, strings, maps, etc., providing a simpler syntax for working with them.

Example:

const arr = ['Alice', 'Bob', 'Charlie'];
for (const name of arr) {
  console.log(name);
}
// Output: "Alice", "Bob", "Charlie"

38. What is the Map object in JavaScript?

Map is a collection of key-value pairs where keys can be of any data type. It maintains the order of elements and provides methods to get, set, and delete entries.

Example:

const map = new Map();
map.set('name', 'Alice');
map.set('age', 25);
console.log(map.get('name')); // "Alice"
console.log(map.has('age')); // true

39. What is the purpose of Array.from() in JavaScript?

Array.from() creates a new array instance from an array-like or iterable object, such as a NodeList or arguments object.

Example:

const nodeList = document.querySelectorAll('p');
const array = Array.from(nodeList);
console.log(array instanceof Array); // true

40. What is the event loop, and how does it work in JavaScript?

The event loop is a core part of JavaScript’s runtime model, responsible for handling asynchronous operations. It continuously checks the call stack and the message queue. When the call stack is empty, it dequeues the first event from the queue and pushes it onto the stack for execution. This process ensures that asynchronous code is executed after the main thread has completed its current tasks.

41. What are generators in JavaScript?

Generators are a special class of functions that can pause and resume their execution. They are defined using the function* syntax and yield control back to the caller with the yield keyword. Generators allow you to produce values lazily, which can be useful for working with large data sets or implementing asynchronous control flows.

Example:

function* generatorFunction() {
  yield 'First';
  yield 'Second';
  yield 'Third';
}
const gen = generatorFunction();
console.log(gen.next().value); // "First"
console.log(gen.next().value); // "Second"

42. What is the difference between Set and WeakSet?

• Set: A collection of unique values where each value may only occur once. It holds values strongly, meaning they are not garbage-collected as long as they exist in the set.
• WeakSet: Similar to Set, but it only holds weak references to its objects. This means objects in a WeakSet can be garbage-collected if there are no other references to them.

Example:

let obj = { a: 1 };
let set = new Set([obj]);
let weakSet = new WeakSet([obj]);

obj = null; // obj is still in the set but can be garbage collected in the weakSet

43. What is a Proxy in JavaScript?

A Proxy is an object that allows you to intercept and customize operations performed on another object (the target). Proxies can be used for tasks like validation, logging, or modifying behavior on the fly.

Example:

const handler = {
  get(target, prop) {
    return prop in target ? target[prop] : `Property ${prop} does not exist`;
  }
};
const target = { name: 'Alice' };
const proxy = new Proxy(target, handler);

console.log(proxy.name); // "Alice"
console.log(proxy.age);  // "Property age does not exist"

44. What is the Reflect API in JavaScript?

Reflect is a built-in object that provides methods for interceptable JavaScript operations. It is complementary to the Proxy object, providing methods to carry out operations like getting, setting, or deleting properties.

Example:

const obj = { name: 'Alice' };
Reflect.set(obj, 'age', 25);
console.log(obj.age); // 25

45. What are Symbols in JavaScript, and how are they used?

A Symbol is a unique and immutable primitive value often used as keys for object properties. Symbols are guaranteed to be unique, making them useful for defining constants or private properties.

Example:

const sym = Symbol('uniqueKey');
const obj = { [sym]: 'secretValue' };
console.log(obj[sym]); // "secretValue"

46. What is the Temporal Dead Zone in JavaScript?

The Temporal Dead Zone (TDZ) refers to the time between entering a scope and the declaration of a let or const variable within that scope. Accessing the variable before its declaration results in a ReferenceError.

Example:

console.log(a); // ReferenceError: Cannot access 'a' before initialization
let a = 10;

47. What is tail call optimization in JavaScript?

Tail call optimization is a feature where the JavaScript engine optimizes recursive functions by eliminating the need for additional stack frames. This optimization occurs when a function’s last action is calling another function (the “tail call”).

Example:

function factorial(n, acc = 1) {
  if (n <= 1) return acc;
  return factorial(n - 1, n * acc); // tail call
}
console.log(factorial(5)); // 120

48. What is the purpose of Object.freeze() in JavaScript?

Object.freeze() prevents new properties from being added to an object, existing properties from being removed, and existing properties from being changed. The object becomes immutable.

Example:

const obj = { name: 'Alice' };
Object.freeze(obj);
obj.name = 'Bob'; // This change will not take effect
console.log(obj.name); // "Alice"

49. How does JavaScript’s async and await work?

async and await are syntactic sugar over promises, making asynchronous code easier to write and understand. An async function always returns a promise, and the await keyword pauses the function’s execution until the promise is resolved or rejected.

Example:

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

Certainly! Here are more advanced JavaScript interview questions with explanations:

50. What is the event loop, and how does it work in JavaScript?

The event loop is a core part of JavaScript’s runtime model, responsible for handling asynchronous operations. It continuously checks the call stack and the message queue. When the call stack is empty, it dequeues the first event from the queue and pushes it onto the stack for execution. This process ensures that asynchronous code is executed after the main thread has completed its current tasks.

51. What are generators in JavaScript?

Generators are a special class of functions that can pause and resume their execution. They are defined using the function* syntax and yield control back to the caller with the yield keyword. Generators allow you to produce values lazily, which can be useful for working with large data sets or implementing asynchronous control flows.

Example:

function* generatorFunction() {
  yield 'First';
  yield 'Second';
  yield 'Third';
}
const gen = generatorFunction();
console.log(gen.next().value); // "First"
console.log(gen.next().value); // "Second"

52. What is the difference between Set and WeakSet?

• Set: A collection of unique values where each value may only occur once. It holds values strongly, meaning they are not garbage-collected as long as they exist in the set.
• WeakSet: Similar to Set, but it only holds weak references to its objects. This means objects in a WeakSet can be garbage-collected if there are no other references to them.

Example:

let obj = { a: 1 };
let set = new Set([obj]);
let weakSet = new WeakSet([obj]);

obj = null; // obj is still in the set but can be garbage collected in the weakSet

53. What is a Proxy in JavaScript?

A Proxy is an object that allows you to intercept and customize operations performed on another object (the target). Proxies can be used for tasks like validation, logging, or modifying behavior on the fly.

Example:

const handler = {
  get(target, prop) {
    return prop in target ? target[prop] : `Property ${prop} does not exist`;
  }
};
const target = { name: 'Alice' };
const proxy = new Proxy(target, handler);

console.log(proxy.name); // "Alice"
console.log(proxy.age);  // "Property age does not exist"

54. What is the Reflect API in JavaScript?

Reflect is a built-in object that provides methods for interceptable JavaScript operations. It is complementary to the Proxy object, providing methods to carry out operations like getting, setting, or deleting properties.

Example:

const obj = { name: 'Alice' };
Reflect.set(obj, 'age', 25);
console.log(obj.age); // 25

55. What are Symbols in JavaScript, and how are they used?

A Symbol is a unique and immutable primitive value often used as keys for object properties. Symbols are guaranteed to be unique, making them useful for defining constants or private properties.

Example:

const sym = Symbol('uniqueKey');
const obj = { [sym]: 'secretValue' };
console.log(obj[sym]); // "secretValue"

56. What is the Temporal Dead Zone in JavaScript?

The Temporal Dead Zone (TDZ) refers to the time between entering a scope and the declaration of a let or const variable within that scope. Accessing the variable before its declaration results in a ReferenceError.

Example:

console.log(a); // ReferenceError: Cannot access 'a' before initialization
let a = 10;

57. What is tail call optimization in JavaScript?

Tail call optimization is a feature where the JavaScript engine optimizes recursive functions by eliminating the need for additional stack frames. This optimization occurs when a function’s last action is calling another function (the “tail call”).

Example:

function factorial(n, acc = 1) {
  if (n <= 1) return acc;
  return factorial(n - 1, n * acc); // tail call
}
console.log(factorial(5)); // 120

58. What is the purpose of Object.freeze() in JavaScript?

Object.freeze() prevents new properties from being added to an object, existing properties from being removed, and existing properties from being changed. The object becomes immutable.

Example:

const obj = { name: 'Alice' };
Object.freeze(obj);
obj.name = 'Bob'; // This change will not take effect
console.log(obj.name); // "Alice"

59. How does JavaScript’s async and await work?

async and await are syntactic sugar over promises, making asynchronous code easier to write and understand. An async function always returns a promise, and the await keyword pauses the function’s execution until the promise is resolved or rejected.

Example:

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

60. What are tagged template literals in JavaScript?

Tagged template literals allow you to parse template literals with a function. The tag function can access the literal parts and the expressions separately.

Example:

function tag(strings, ...values) {
  console.log(strings);
  console.log(values);
}

const name = 'Alice';
tag`Hello, ${name}!`; // ["Hello, ", "!"], ["Alice"]

🎯Conclusion

JavaScript is a powerful and versatile language that continues to evolve, making it essential for developers to stay updated with its features and best practices. Whether you’re preparing for an interview or aiming to deepen your understanding, exploring these advanced JavaScript concepts will enhance your problem-solving skills and coding efficiency.

Mastering topics such as the event loop, promises, and ES6+ features will not only prepare you for challenging interview questions but also make you a more proficient developer. Remember, the key to success in JavaScript is continuous learning and practical application. Keep experimenting with new patterns, optimizing your code, and staying curious—this is the path to becoming a top-tier JavaScript developer.

You can visit our website Tutorial Temple for more related topic. Thank You.

🤔💭FAQ (Frequently Asked Questions)