Top JavaScript Interview Questions #

1. What is JavaScript? #

JavaScript is a high-level, interpreted programming language primarily used for adding interactivity and dynamic behavior to web pages. It is commonly used in web development for both front-end and back-end tasks.

2. What are the key features of JavaScript? #

JavaScript features include:

• Dynamic Typing: Variables are not bound to specific data types.
• Closures: Functions can maintain their own scope even after they’re executed.
• Prototype-based Object Orientation: Objects can inherit properties and methods from other objects.
• First-class Functions: Functions can be treated as objects, passed as arguments, and returned from other functions.
• Asynchronous Programming: Supports non-blocking operations through callbacks and promises.
• Event-Driven Programming: Used to respond to user actions and other events.

3. How does JavaScript differ from Java? #

Despite the similar names, JavaScript and Java are quite different:

• Origin: JavaScript was developed by Netscape for web scripting, while Java was created by Sun Microsystems as a general-purpose programming language.
• Syntax: JavaScript’s syntax is influenced by C and C++, whereas Java’s syntax is similar to C++.
• Type System: JavaScript has dynamic typing, while Java has static typing.
• Platform: Java can run on a wide range of platforms using the Java Virtual Machine (JVM), while JavaScript is primarily used within web browsers.
• Usage: JavaScript is mainly used for web development, while Java is used for various applications including web, desktop, and mobile.

4. What is the DOM? #

The Document Object Model (DOM) is a programming interface provided by browsers. It represents the structure of HTML or XML documents as objects, allowing scripts to interact with and manipulate the content and structure of web pages.

5. Explain closures in JavaScript. #

Closures are functions that “remember” their lexical scope even when executed outside that scope. They allow data encapsulation and can be used to create private variables and functions. Closures are often used in scenarios like callbacks and maintaining state in asynchronous operations.

6. How does prototypal inheritance work in JavaScript? #

In JavaScript, objects can inherit properties and methods from other objects through their prototypes. When a property or method is accessed on an object, JavaScript searches the object itself and then its prototype chain until it finds the property/method or reaches the end of the chain (usually the Object.prototype).

7. What is the purpose of the this keyword in JavaScript? #

The this keyword refers to the current execution context. In a method, it refers to the object that the method is called on. Its value can change based on how a function is called: in the global scope, within an object, or as a standalone function.

8. How do you handle asynchronous operations in JavaScript? #

Asynchronous operations in JavaScript are managed using callbacks, promises, and async/await.

• Callbacks: Functions passed as arguments to be executed after an operation completes.
• Promises: Objects representing the eventual completion or failure of an asynchronous operation.
• Async/await: Syntactic sugar built on top of promises, making asynchronous code look more synchronous.

9. What are arrow functions? #

Arrow functions are a concise way to write functions in JavaScript. They have a shorter syntax compared to traditional function expressions and inherit the this value from the surrounding code.

10. How can you mitigate the risk of callback hell? #

Callback hell, also known as the “Pyramid of Doom,” occurs when multiple nested callbacks make code hard to read and maintain. To mitigate this, you can use techniques like named functions, promises, or async/await to create more organized and readable asynchronous code.

11. What is event delegation in JavaScript? #

Event delegation is a technique where you attach a single event listener to a common ancestor of multiple elements instead of attaching individual listeners to each element. This is useful for improving performance and dynamically handling events for elements that are added or removed from the DOM.

12. What are the differences between null and undefined? #

• undefined: A variable that has been declared but has not been assigned a value is undefined. It’s also the default value for function parameters that are not provided.
• null: It’s an intentional absence of any object value. It can be assigned to a variable to represent no value or no object.

13. Explain the concept of hoisting. #

Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their containing scope during compilation. However, only the declarations are hoisted, not the initializations. This means you can use variables and functions before they’re declared, although it’s considered good practice to declare them first.

14. How does the localStorage differ from sessionStorage? #

• localStorage: Data stored in localStorage persists even after the browser is closed and has no expiration time unless explicitly cleared by the user or script.
• sessionStorage: Data stored in sessionStorage is available only for the duration of the page session. It is removed when the user closes the tab or window.

15. What is the purpose of the bind() method? #

The bind() method is used to create a new function that, when called, has its this value set to a specific value regardless of how the function is actually called. It’s commonly used to bind a function to a particular context, especially in event handlers or callbacks.

16. How can you copy an object in JavaScript? #

To copy an object in JavaScript, you can use techniques like:

• Object.assign(): Creates a shallow copy of an object.
• Spread Operator (...): Can be used to create a shallow copy of an object.
• JSON.parse() and JSON.stringify(): Convert the object to JSON and then parse it back to create a copy. Note that this method works well for simple objects but may not handle more complex cases.

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

The map() method is used to create a new array by applying a provided function to each element of the calling array. It’s a non-mutating method and is often used for transforming data in arrays.

18. Explain the Same-Origin Policy and how it relates to JavaScript. #

The Same-Origin Policy is a security feature implemented by web browsers to prevent web pages from making requests to a different domain than the one that served the web page. This helps mitigate cross-site scripting (XSS) and other security vulnerabilities.

19. What is the purpose of a closure in JavaScript? #

Closures are used to encapsulate and maintain the state of a function even after it has finished executing. They are often used to create private variables, currying, and in scenarios where you need to preserve a certain context for a function.

20. How do you handle errors in JavaScript? #

JavaScript provides the try...catch statement to handle errors. Code that might throw an exception is placed within the try block, and if an exception is thrown, it’s caught and handled in the catch block. This prevents the entire script from crashing due to an error.