Top 20 JavaScript Interview Questions for Experienced Developers

Introduction

JavaScript is one of the most widely used programming languages, especially in web development. As an experienced developer, it’s important to be well-versed in JavaScript and be prepared for technical interviews. In this blog post, we will discuss the top 20 JavaScript interview questions that are commonly asked during interviews for experienced developers.

1. What is JavaScript?

JavaScript is a high-level, interpreted programming language that is primarily used for creating interactive web pages. It is a versatile language that can be used both on the client-side and server-side.

2. What are the key features of JavaScript?

JavaScript has several key features, including:

• Object-oriented programming
• Dynamic typing
• Prototype-based inheritance
• First-class functions

3. What is the difference between null and undefined?

Null and undefined are both JavaScript data types that represent the absence of a value. However, null is an assignment value that indicates the absence of an object, while undefined means a variable has been declared but has not been assigned a value.

4. What are closures in JavaScript?

Closures are functions that have access to variables from an outer function that has already returned. They allow you to preserve the state of a function and can be used to create private variables and functions.

5. Explain the concept of hoisting in JavaScript.

Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their containing scope during the compilation phase. This means that you can use variables and functions before they are declared.

6. What is the difference between ‘==’ and ‘===’ operators?

The ‘==’ operator compares values after performing type coercion, while the ‘===’ operator compares both values and types without performing type coercion. In most cases, it is recommended to use the ‘===’ operator for strict equality checks.

7. What is event delegation in JavaScript?

Event delegation is a technique where you attach an event listener to a parent element instead of individual child elements. This allows you to handle events on dynamically added elements and improves performance by reducing the number of event listeners.

8. How does prototypal inheritance work in JavaScript?

JavaScript uses prototypal inheritance, where objects can inherit properties and methods from other objects. Each object has a prototype object, and when a property or method is accessed, JavaScript looks up the prototype chain until it finds the property or method.

9. What is the purpose of the ‘this’ keyword in JavaScript?

The ‘this’ keyword refers to the object that the function is executing in. It allows you to access and manipulate properties of the object within the function. The value of ‘this’ is determined by how a function is called.

10. What is the difference between call, apply, and bind?

Call, apply, and bind are methods that allow you to control the value of ‘this’ in a function.

• The ‘call’ method invokes a function with a specified ‘this’ value and arguments provided individually.
• The ‘apply’ method invokes a function with a specified ‘this’ value and arguments provided as an array.
• The ‘bind’ method returns a new function with a specified ‘this’ value and any additional arguments provided.

11. Explain the concept of event bubbling in JavaScript.

Event bubbling is a phenomenon in which an event triggered on a child element is propagated up through its parent elements. This allows you to handle events on multiple elements using a single event listener.

12. What is the purpose of the ‘use strict’ directive in JavaScript?

The ‘use strict’ directive enables strict mode in JavaScript, which enforces stricter rules and provides better error handling. It helps to prevent common mistakes and makes code more reliable.

13. How do you handle asynchronous operations in JavaScript?

Asynchronous operations in JavaScript can be handled using callbacks, promises, or async/await. Callbacks are the traditional approach, while promises and async/await provide more readable and manageable code.

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

‘let’ and ‘const’ are block-scoped variables introduced in ES6, while ‘var’ is function-scoped. ‘let’ allows reassignment, while ‘const’ is used for variables that should not be reassigned. ‘var’ has some quirks, such as hoisting and the ability to be redeclared.

15. How do you handle errors in JavaScript?

Errors in JavaScript can be handled using try/catch blocks. The ‘try’ block contains the code that may throw an error, and the ‘catch’ block is used to handle the error and provide fallback logic.

16. What are arrow functions in JavaScript?

Arrow functions are a concise syntax for writing function expressions in JavaScript. They have a shorter syntax and lexically bind the ‘this’ value, which can be useful in certain scenarios.

17. What are the different types of events in JavaScript?

JavaScript has several types of events, including:

• Mouse events (click, mouseover, etc.)
• Keyboard events (keydown, keyup, etc.)
• Form events (submit, change, etc.)
• Document events (DOMContentLoaded, load, etc.)

18. How do you handle cross-origin requests in JavaScript?

Cross-origin requests can be handled using CORS (Cross-Origin Resource Sharing). CORS allows servers to specify who can access their resources, preventing unauthorized access from different origins.

19. 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. This makes asynchronous code appear more synchronous and easier to read.

20. How do you debug JavaScript code?

JavaScript code can be debugged using browser developer tools, such as the Chrome DevTools. These tools provide features like breakpoints, step-through debugging, and console logging to help identify and fix issues in the code.

Conclusion

These top 20 JavaScript interview questions cover a range of important concepts and techniques that experienced developers should be familiar with. By understanding and practicing these questions, you can increase your chances of performing well in JavaScript technical interviews and showcase your expertise in the language.

Remember to not only memorize the answers but also understand the underlying concepts behind each question. This will enable you to apply your knowledge effectively in real-world scenarios.