JavaScript Interview Questions and Answers for freshers

1. What is JavaScript?

   • Answer: JavaScript is a high-level, interpreted programming language primarily used to make websites interactive. It's also used for server-side programming (Node.js), mobile app development (React Native), and more.

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

   • Answer: == performs loose equality, comparing values after type coercion. === performs strict equality, comparing both value and type without coercion. For example, 1 == "1" is true, but 1 === "1" is false.

3. Explain the concept of hoisting in JavaScript.

   • Answer: Hoisting is a JavaScript mechanism where declarations of variables and functions are moved to the top of their scope before code execution. This means you can use a variable or function before it's declared in the code, but only if it's declared using var (or implicitly with no declaration - though this is generally bad practice), not let or const.

4. What are closures in JavaScript?

   • Answer: A closure is a function that has access to variables from its surrounding lexical environment (the environment in which it was created), even after that environment has finished executing. This allows inner functions to "remember" and access variables from their outer functions.

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

   • Answer: var has function scope (or global scope if not inside a function) and can be re-declared and updated. let and const have block scope (within {}). let can be updated, but const cannot be reassigned after initialization. It's best practice to use let for variables that might change and const for variables that won't.

6. Explain the concept of `this` in JavaScript.

   • Answer: The value of this depends on how the function is called. In a regular function call, this points to the global object (window in browsers). In methods, this refers to the object the method belongs to. Arrow functions do not have their own this; they inherit it from the surrounding scope.

7. What are JavaScript objects?

   • Answer: JavaScript objects are collections of key-value pairs. The keys are strings (or Symbols), and the values can be any data type. Objects are used to represent structured data.

8. What are prototypes in JavaScript?

   • Answer: Prototypes are the mechanism by which JavaScript objects inherit properties and methods from other objects. Every object has a prototype, which can be another object. When you try to access a property on an object, and it's not found directly on the object, JavaScript searches the prototype chain until the property is found or the end of the chain is reached.

9. Explain how to create a new object in JavaScript.

   • Answer: You can create new objects using object literals ({}), the new Object() constructor, or using classes (ES6 and later).

10. What are the different data types in JavaScript?

    • Answer: JavaScript has several data types, including: Number, String, Boolean, null, undefined, Symbol, BigInt, and Object (which includes Arrays, Dates, etc.).

11. What is a Promise in JavaScript?

    • Answer: A Promise is an object representing the eventual completion (or failure) of an asynchronous operation, and its resulting value. It can be in one of three states: pending, fulfilled, or rejected.

12. Explain async/await.

    • Answer: Async/await is a syntactic sugar for working with Promises. async makes a function return a Promise, and await pauses execution until a Promise resolves.

13. What is event delegation?

    • Answer: Event delegation is a technique where you attach an event listener to a parent element, and it handles events that bubble up from its children. This is more efficient than attaching listeners to many individual child elements.

14. What are some common JavaScript frameworks or libraries?

    • Answer: Popular choices include React, Angular, Vue.js, jQuery (less common now), Node.js (server-side).

15. How do you handle errors in JavaScript?

    • Answer: Using try...catch blocks to handle exceptions, and finally for cleanup code.

16. Explain the concept of Immediately Invoked Function Expressions (IIFEs).

    • Answer: IIFEs are functions that are defined and immediately called. They're often used to create private scopes and avoid polluting the global namespace.

17. What is JSON?

    • Answer: JSON (JavaScript Object Notation) is a lightweight data-interchange format. It's easy for humans to read and write, and easy for machines to parse and generate.

18. How do you debug JavaScript code?

    • Answer: Using browser developer tools (console logging, breakpoints, stepping through code), linters, and debugging tools like Node's debugger.

19. What is the DOM?

    • Answer: The Document Object Model (DOM) is a programming interface for HTML and XML documents. It represents the page so that programs can change the document structure, style, and content.

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