Top 10 JavaScript Interview Questions and Answers

 As JavaScript continues to dominate web development, preparing for an interview requires a solid grasp of its concepts. Here are ten common JavaScript interview questions along with detailed answers to help you get ready.

1. What are the data types supported by JavaScript?

Answer: JavaScript supports several data types:

• Primitive Types: Undefined, Null, Boolean, Number, String, Symbol (ES6), and BigInt (ES11).
• Reference Types: Objects, Arrays, and Functions.

Primitive types are immutable, while reference types are mutable and can hold collections of data.

2. What is the difference between == and ===?

Answer:

• == (Equality Operator): Compares two values for equality after converting them to a common type (type coercion).
• === (Strict Equality Operator): Compares both value and type without type coercion. It’s generally recommended to use === to avoid unexpected results.

3. Explain the concept of closures in JavaScript.

Answer: A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope. This means that inner functions can access variables from their outer function, even after the outer function has finished executing. Closures are often used for data privacy and creating function factories.

4. What are Promises, and how do they work?

Answer: A Promise is an object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value. It can be in one of three states:

• Pending: The initial state, neither fulfilled nor rejected.
• Fulfilled: The operation completed successfully.
• Rejected: The operation failed.

You can handle results using .then() for fulfilled promises and .catch() for rejected ones.

5. What is event delegation in JavaScript?

Answer: Event delegation is a technique in which a single event listener is added to a parent element instead of individual listeners for each child element. This takes advantage of event bubbling, allowing you to manage events for dynamic elements efficiently and improve performance by reducing the number of event listeners.

6. What is the difference between null and undefined?

Answer:

• null: A value assigned to indicate the absence of any object value. It is an intentional absence.
• undefined: A variable that has been declared but has not yet been assigned a value. It represents an uninitialized state.

7. What are arrow functions, and how do they differ from regular functions?

Answer: Arrow functions are a concise syntax for writing function expressions in JavaScript. They do not have their own this context, which means they inherit this from the enclosing scope, making them particularly useful in scenarios like callbacks where maintaining the context is essential.

8. What is the purpose of the this keyword in JavaScript?

Answer: The this keyword refers to the context in which a function is called. Its value can vary based on how a function is invoked:

• In a method, this refers to the object it belongs to.
• In a function, it refers to the global object (or undefined in strict mode).
• In an arrow function, it inherits this from the enclosing lexical scope.

9. Explain the concept of hoisting in JavaScript.

Answer: Hoisting is JavaScript's behavior of moving variable and function declarations to the top of their containing scope during the compilation phase. This means you can reference functions and variables before they are declared in the code. However, only declarations are hoisted, not initializations.

10. What are modules in JavaScript?

Answer: Modules are reusable pieces of code that export specific functions, objects, or values and can be imported into other JavaScript files. They help in organizing code, avoiding global scope pollution, and promoting encapsulation. ES6 introduced native module support using the import and export syntax.