JavaScript Interview Questions & Answers

Basics of JavaScript:

1. What is JavaScript?

   • Answer: JavaScript is a high-level, interpreted programming language that is widely used for creating interactive and dynamic web pages.

2. What are the key features of JavaScript?

   • Answer: Key features include being lightweight, interpreted, object-oriented, and supporting both procedural and functional programming paradigms.

3. Explain the difference between null and undefined in JavaScript.

   • Answer: null is an intentional absence of any object value, while undefined indicates a variable has been declared but has not been assigned a value.

4. What is the role of the typeof operator in JavaScript?

   • Answer: The typeof operator is used to determine the data type of a variable or an expression.

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

   • Answer: The this keyword refers to the current execution context, usually the object that owns the method or the context in which a function is called.

Variables and Data Types:

6. How do you declare a variable in JavaScript?

   • Answer: You can declare a variable using the var, let, or const keyword, followed by the variable name.

7. Explain the differences between var, let, and const.

   • Answer: var has function scope, let has block scope, and const is used for constants and cannot be reassigned.

8. What are the basic data types in JavaScript?

   • Answer: Primitive data types include number, string, boolean, null, undefined, symbol, and bigint.

9. How do you check the data type of a variable?

   • Answer: Use the typeof operator. For example: typeof variableName.

10. What is type coercion in JavaScript?

    • Answer: Type coercion is the automatic conversion of values between different data types during runtime.

Control Flow and Loops:

11. Explain the if...else statement in JavaScript.

    • Answer: The if...else statement is used for conditional execution of code. If the condition is true, the code inside the if block is executed; otherwise, the code inside the else block is executed.

12. What is a switch statement, and how does it work?

    • Answer: A switch statement is used to perform different actions based on different conditions. It evaluates an expression and compares it with multiple case values.

13. How do you use a for loop in JavaScript?

    • Answer: A for loop is used for iterating over a block of code a specified number of times. It consists of an initialization, a condition, and an update statement.

14. Explain the while loop in JavaScript.

    • Answer: The while loop is used to repeatedly execute a block of code as long as a specified condition is true.

15. What is the purpose of the break statement in a loop?

    • Answer: The break statement is used to exit a loop prematurely. It is often used within a loop's body to terminate the loop based on a certain condition.

Functions:

16. How do you declare a function in JavaScript?

    • Answer: You can declare a function using the function keyword, followed by the function name and parameters.

17. Explain the difference between function declarations and function expressions.

    • Answer: Function declarations are hoisted, meaning they can be used before they are declared. Function expressions are not hoisted and must be declared before use.

18. What is a callback function?

    • Answer: A callback function is a function passed as an argument to another function, which is then invoked inside the outer function.

19. What is a closure in JavaScript?

    • Answer: A closure is the combination of a function and the lexical environment within which that function was declared. It allows a function to access variables from its outer scope even after the outer function has finished execution.

20. What is the purpose of the arguments object in a JavaScript function?

    • Answer: The arguments object is an array-like object available inside all functions. It contains the values of the arguments passed to the function.

Objects and Arrays:

21. How do you create an object in JavaScript?

    • Answer: You can create an object using object literal notation {} or the Object constructor.

22. What is the difference between an object and an array in JavaScript?

    • Answer: An object is an unordered collection of key-value pairs, while an array is an ordered collection of values with numeric indices.

23. How do you access properties of an object in JavaScript?

    • Answer: You can access properties using dot notation (object.property) or bracket notation (object['property']).

24. Explain the concept of prototypal inheritance in JavaScript.

    • Answer: Prototypal inheritance allows objects to inherit properties and methods from other objects. Each object has a prototype, and objects can inherit from other objects through their prototype chain.

25. What is an array in JavaScript?

    • Answer: An array is a special type of object used for storing and manipulating ordered collections of values.

DOM Manipulation:

26. What is the DOM (Document Object Model) in JavaScript?
    • Answer: The DOM is a programming interface for web documents. It represents the document as a tree of objects, allowing scripts to interact with and manipulate the content and

structure of a web page.

27. How do you select an element by its ID using JavaScript?

    • Answer: You can use document.getElementById('elementId') to select an element by its ID.

28. Explain event delegation in JavaScript.

    • Answer: Event delegation is a technique where a single event listener is attached to a common ancestor of multiple elements. This allows handling events for all the elements in a more efficient way.

29. What is the purpose of the addEventListener method in JavaScript?

    • Answer: The addEventListener method is used to attach an event handler function to an element, allowing the element to respond to events.

30. How do you modify the content of an HTML element using JavaScript?

    • Answer: You can modify the content of an element using the innerHTML property or by manipulating its text content using the textContent property.

Asynchronous JavaScript:

31. What is asynchronous programming in JavaScript?

    • Answer: Asynchronous programming allows executing code out of sequence, without blocking the main thread. It is commonly used for handling tasks such as fetching data, timers, and event handling.

32. Explain the concept of callbacks in asynchronous JavaScript.

    • Answer: Callbacks are functions passed as arguments to other functions. In asynchronous JavaScript, callbacks are often used to handle the result of asynchronous operations.

33. What is the purpose of the Promise object in JavaScript?

    • Answer: The Promise object represents the eventual completion or failure of an asynchronous operation and its resulting value.

34. Explain the difference between callbacks and Promises.

    • Answer: Callbacks are functions passed as arguments to other functions, while Promises provide a cleaner and more structured way to handle asynchronous operations, making code more readable and maintainable.

35. How does async/await improve asynchronous code in JavaScript?

    • Answer: async/await is a syntactic sugar for working with Promises. It makes asynchronous code look and behave more like synchronous code, improving readability and maintainability.

Error Handling and Debugging:

36. How do you handle errors in JavaScript?

    • Answer: You can use try...catch blocks to handle errors in JavaScript. Additionally, the catch block can be used to log or handle the error.

37. Explain the concept of hoisting in JavaScript.

    • Answer: Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their containing scope during the compilation phase.

38. What is the purpose of the throw statement in JavaScript?

    • Answer: The throw statement is used to create a custom error object and signal that an error has occurred. It is often used in conjunction with try...catch blocks.

39. How can you debug JavaScript code?

    • Answer: Debugging can be done using browser developer tools, console.log() statements, breakpoints, and by employing tools like debugger.

40. What is the role of the console object in JavaScript?

    • Answer: The console object provides methods for logging information, warnings, errors, and other messages to the console, facilitating debugging.

ES6 and Modern JavaScript:

41. What are some new features introduced in ECMAScript 6 (ES6)?

    • Answer: ES6 introduced features such as arrow functions, let and const for variable declarations, template literals, destructuring, classes, and the Promise object.

42. Explain the concept of arrow functions in JavaScript.

    • Answer: Arrow functions are a concise syntax for writing function expressions. They have a shorter syntax and do not bind their own this value.

43. What is destructuring in JavaScript?

    • Answer: Destructuring is a feature that allows extracting values from arrays or objects and assigning them to variables in a more concise way.

44. How do you use template literals in JavaScript?

    • Answer: Template literals use backticks () and allow embedding expressions within string literals using ${expression}`.

45. What are classes in JavaScript, and how do they differ from prototypes?

    • Answer: Classes in JavaScript are a syntactic sugar for working with prototypes. They provide a more familiar class-based syntax but are still based on prototype inheritance.

JavaScript Frameworks and Libraries:

46. What is jQuery, and why is it used in JavaScript development?

    • Answer: jQuery is a fast and lightweight JavaScript library. It simplifies tasks like DOM manipulation, event handling, and AJAX requests, providing a more convenient API.

47. Explain the role of React in JavaScript development.

    • Answer: React is a JavaScript library for building user interfaces. It facilitates the creation of reusable UI components and efficiently manages the state of an application.

48. What is the purpose of Angular in JavaScript development?

    • Answer: Angular is a comprehensive front-end framework for building dynamic web applications. It provides tools and services for various aspects of web development, including data binding, dependency injection, and routing.