JavaScript Interview Questions Best For Prepare in 2025

What Is JavaScript?

JavaScript is a versatile, high-level programming language used primarily for creating interactive and dynamic web content. As one of the core technologies of the web, along with HTML and CSS, JavaScript enables features such as real-time updates, animations, and user interactions. It is widely supported by all modern web browsers and can be used on both the client-side (frontend) and server-side (backend) in environments such as Node.js. JavaScript’s flexibility and extensive libraries make it a popular choice for building web applications, mobile applications, and even games.

JavaScript Interview Questions 2025

JavaScript interview questions in 2025 focus on a mix of foundational concepts, modern practices, and new features introduced in recent versions of ECMAScript. Common topics include:

1. Basic Concepts: Interviewers often test knowledge of variables (let, const, var), data types, functions, scopes, and closures.
2. DOM Manipulation: Questions may involve manipulating the Document Object Model (DOM) using JavaScript, handling events, and understanding browser behaviors.
3. Asynchronous Programming: Expect questions on promises, async/await, and how JavaScript handles asynchronous operations and concurrency with the event loop.
4. ES6+ Features: With the continuous updates to JavaScript, understanding newer features like arrow functions, destructuring, spread/rest operators, template literals, and modules is key.
5. Object-Oriented JavaScript: Interviewers may ask about classes, inheritance, and prototypes to assess your understanding of object-oriented principles in JavaScript.
6. Error Handling: Questions about try-catch blocks, custom error handling, and debugging techniques are common.
7. Performance Optimization: Expect discussions around optimizing code, memory management, and common performance bottlenecks in JavaScript.
8. Frameworks and Libraries: Familiarity with popular libraries like React, Angular, and Vue.js, as well as understanding how JavaScript interacts with these, is often tested.
9. JavaScript Design Patterns: Knowledge of design patterns such as module, singleton, and observer patterns might be examined.

These topics test both your problem-solving abilities and your understanding of how JavaScript works at a deeper level, including the newest updates and best practices for 2025.

Interview Questions And Answers 2025

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

Answer:

• null is an assignment value that represents the intentional absence of any object value. It is explicitly assigned to a variable to indicate “no value” or “empty value.”
• undefined is a type and also represents the absence of a value. It is the default value assigned to a variable that has been declared but not initialized.

In summary:

• null is an object (although a bug in JavaScript’s design), and it is explicitly set.
• undefined is a type and is automatically assigned to uninitialized variables.

2. What is a closure in JavaScript?

Answer: A closure is a function that “remembers” its lexical scope, even when the function is executed outside that scope. In simpler terms, it allows a function to access variables from an outer function even after the outer function has finished executing.

Here, the inner function is a closure because it retains access to the outerVar variable even after the outer function has finished executing. This allows for powerful patterns, such as data encapsulation and private variables.