Top JavaScript Interview Questions and Answers

Here are top JavaScript interview questions,

1. What is JavaScript?

JavaScript is a high-level, interpreted programming language that is primarily used for web development. It allows for dynamic content, interactivity, and client-side functionality in web applications.

 

2. What are the data types in JavaScript?

JavaScript has six primitive data types: string, number, boolean, null, undefined, and symbol (added in ES6). It also has a complex data type called object, which includes arrays and functions.

 

3. What is the difference between "null" and "undefined" in JavaScript?

"null" represents the intentional absence of any object value, while "undefined" represents the absence of a value or uninitialized variable.

 

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

"let" and "const" are block-scoped variables introduced in ES6, while "var" is function-scoped. "let" allows reassignment, "const" is for constant values, and "var" has hoisting behavior.

 

5. What is the difference between "==" and "===" in JavaScript?

"==" performs type coercion and checks for equality after converting the operands to a common type, while "===" checks for strict equality without type conversion.

 

6. What are closures in JavaScript?

A closure is a combination of a function and the lexical environment within which that function was declared. It allows the function to retain access to variables from its outer scope even after the outer function has finished executing.

 

7. What is the event loop in JavaScript?

The event loop is a mechanism in JavaScript that handles asynchronous callbacks. It ensures that events, such as user interactions and network requests, are executed in a non-blocking manner.

 

8. What is the difference between "null," "undefined," and "NaN"?

"null" represents the intentional absence of any object value, "undefined" represents the absence of a value or uninitialized variable, and "NaN" (Not a Number) represents an invalid or unrepresentable numeric value.

 

9. What is hoisting in JavaScript?

Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their respective scopes during the compilation phase. However, only the declarations are hoisted, not the initializations.

 

10. What are the different ways to create objects in JavaScript?

Objects can be created in JavaScript using object literals, the "new" keyword with constructor functions, and the "class" syntax introduced in ES6.

 

11. What is the difference between "let" and "var" in JavaScript?

"let" and "var" are both used for variable declarations, but "let" has block scope and is not hoisted, while "var" has function scope and is hoisted.

 

12. What are the different types of error handling in JavaScript?

JavaScript offers try-catch blocks for handling exceptions, the "throw" statement for throwing custom errors, and the "finally" block for executing code regardless of whether an exception occurred.

 

13. What is the "this" keyword in JavaScript?

The "this" keyword refers to the current context or object on which a function is being executed. Its value is determined by how the function is called.

 

14. What are arrow functions in JavaScript?

Arrow functions are a concise syntax introduced in ES6 for creating anonymous functions. They have a shorter syntax, don't bind their own "this" value, and do not have their own "arguments" object.

 

15. What is the difference between "const" and "readonly" in JavaScript?

In JavaScript, "const" is used to declare constants that cannot be reassigned, while "readonly" is a TypeScript modifier used for class properties that can only be set during initialization or within the constructor.

 

16. What is the purpose of the "async" and "await" keywords in JavaScript?

The "async" keyword is used to define an asynchronous function, and the "await" keyword is used to pause the execution of an asynchronous function until a promise is resolved or rejected.

 

17. What are promises in JavaScript?

Promises are a way to handle asynchronous operations in JavaScript. They represent the eventual completion or failure of an asynchronous operation and allow chaining of multiple asynchronous actions.

 

18. What is the difference between "localStorage" and "sessionStorage" in JavaScript?

Both "localStorage" and "sessionStorage" are web storage APIs for storing data on the client side, but "localStorage" persists even when the browser is closed, while "sessionStorage" is cleared when the browser session ends.

 

19. How do you clone an object in JavaScript?

One way to clone an object in JavaScript is by using the spread operator ({...}) to create a shallow copy. For deep cloning, you can use libraries like Lodash or implement a custom recursive function.

 

20. What is the difference between "synchronous" and "asynchronous" in JavaScript?

Synchronous operations block the execution until the operation is completed, while asynchronous operations allow the program to continue executing while the operation is being performed, typically using callbacks, promises, or async/await.

Above are few top JavaScript interview questions. Remember to prepare and expand on these answers.

Good luck with your interview!  👍