Top 30 JavaScript Interview Questions and Answers for 2024

Level-1: Basic JavaScript Interview Questions

1. Is JavaScript Single-Threaded?

Yes, JavaScript is single-threaded, meaning it can execute only one task at a time. However, it uses the event loop to handle asynchronous operations, allowing it to perform non-blocking tasks.

2. Explain the Main Components of the JavaScript Engine and How It Works.

The main components of a JavaScript engine include the memory heap and the call stack. The engine processes code by compiling it into machine code and executing it through the call stack, managing memory allocation and garbage collection in the heap.

3. Explain the Event Loop in JavaScript and How It Helps in Asynchronous Programming.

The event loop allows JavaScript to perform non-blocking operations by offloading tasks to the browser, thus enabling asynchronous programming. It continuously checks the call stack and the message queue, executing tasks in a non-blocking manner.

4. Difference Between var, let, and const?

var: Function-scoped, can be redeclared, hoisted.
let: Block-scoped, cannot be redeclared, not hoisted.
const: Block-scoped, cannot be redeclared, must be initialized at declaration.

5. Different Data Types in JavaScript?

JavaScript has primitive data types such as String, Number, Boolean, Null, Undefined, Symbol, and BigInt, as well as non-primitive types like Objects.

6. What is a Callback Function and Callback Hell?

A callback function is a function passed into another function as an argument, executed after the completion of the other function. Callback hell refers to the pyramid-like structure of nested callbacks, making code difficult to read and maintain.

7. What is a Promise and Promise Chaining?

A promise represents a value that may be available now, in the future, or never. Promise chaining involves linking multiple promises together, allowing for sequential asynchronous operations.

8. What is async/await?

async and await are keywords that simplify working with promises, making asynchronous code look and behave like synchronous code. async functions return promises, and await pauses the execution until the promise resolves.

9. What is the Difference Between == and === Operators?

==: Loose equality, compares values after type conversion.
===: Strict equality, compares values without type conversion.

10. Different Ways to Create an Object in JavaScript?

Object literals
Constructor functions
Object.create()
ES6 classes

11. What is the Rest and Spread Operator?

Rest (...): Collects all remaining elements into an array.
Spread (...): Expands elements of an array or object.

12. What is a Higher-Order Function?

A higher-order function is a function that takes one or more functions as arguments or returns a function as its result.

Level-2: Intermediate JavaScript Interview Questions

13. What is Closure? What are the Use Cases of Closures?

A closure is a function that has access to its outer function’s scope even after the outer function has returned. Closures are used for data privacy and creating function factories.

14. Explain the Concept of Hoisting in JavaScript.

Hoisting is JavaScript’s default behavior of moving declarations to the top of the current scope, allowing functions and variables to be used before they are declared.

15. What is a Temporal Dead Zone?

The temporal dead zone is the period between entering a scope and the point at which a variable declared with let or const is initialized, during which the variable cannot be accessed.

16. What is a Prototype Chain? and `Object.create()` Method?

The prototype chain is a mechanism by which objects inherit properties and methods from other objects. Object.create() creates a new object with the specified prototype object and properties.

17. What is the Difference Between Call, Apply, and Bind Methods?

Call: Invokes a function with a given this value and arguments.
Apply: Similar to call but takes arguments as an array.
Bind: Returns a new function, permanently binding this to the specified value.

18. What are Lambda or Arrow Functions?

Arrow functions are a shorter syntax for writing functions, introduced in ES6. They do not have their own this context and cannot be used as constructors.

19. What is the Currying Function?

Currying is the process of transforming a function that takes multiple arguments into a series of functions that each take a single argument.

20. What are the Features of ES6?

ES6 introduced features like let and const, arrow functions, template literals, default parameters, destructuring, modules, and classes.

Level-3: Expert JavaScript Interview Questions

21. What is Execution Context, Execution Stack, Variable Object, Scope Chain?

The execution context is an environment where JavaScript code is executed. The execution stack is a stack of execution contexts. The variable object contains function arguments, variables, and inner functions. The scope chain is used to resolve variable access.

22. What is the Priority of Execution of Callback, Promise, setTimeout, process.nextTick()?

process.nextTick(): Highest priority, executed before the next event loop tick.
Promise callbacks: Microtasks, executed after the current synchronous code.
setTimeout(): Macrotasks, executed after all microtasks.

23. What is the Factory Function and Generator Function?

Factory Function: A function that returns a new object instance.
Generator Function: A function that can pause and resume its execution using the yield keyword.

24. Different Ways to Clone (Shallow and Deep Copy of Object) an Object?

Shallow Copy: Object.assign(), spread operator.
Deep Copy: JSON.parse(JSON.stringify()), deep cloning libraries.

25. How to Make an Object Immutable? (`seal` and `freeze` Methods)

Object.seal(): Prevents adding or removing properties.
Object.freeze(): Prevents adding, removing, or modifying properties.

26. What is Event Flow, Event Bubbling, and Event Capturing?

Event Flow: The order in which events are received by the target elements.
Event Bubbling: Event starts from the target element and bubbles up to the root.
Event Capturing: Event starts from the root and captures down to the target element.

27. What is Event Delegation?

Event delegation allows a single event handler to manage events for multiple elements by taking advantage of event bubbling.

28. What are Server-Sent Events?

Server-Sent Events (SSE) allow a server to push updates to the client over a single HTTP connection, used for real-time notifications.

29. What is a Web Worker or Service Worker in JavaScript?

Web Worker: Runs scripts in the background, independently of the user interface.
Service Worker: Acts as a proxy between the web application and the network, enabling offline capabilities and push notifications.

30. How to Compare 2 JSON Objects in JavaScript?

To compare two JSON objects, convert them to strings using JSON.stringify() and compare the strings. For deep comparison, use a deep equality checking function.

Conclusion

In conclusion, mastering these top 30 JavaScript interview questions and answers will significantly boost your confidence and performance in any JavaScript interview. For more in-depth tutorials and interview preparation tips, subscribe to our newsletter and stay updated with the latest trends in JavaScript development.

Top 30 JavaScript Interview Questions and Answers for 2024