Top 40 JavaScript Interview Questions And Answers For Fresher In 2024

Introduction

JavaScript, a versatile and dynamic programming language, serves as the backbone of interactive web development. Empowering web pages with functionality and responsiveness, JavaScript enables developers to create engaging user experiences. Its flexibility, ranging from client-side scripting to server-side development, has made it an integral part of modern web applications. With its ever-evolving ecosystem and broad community support, JavaScript continues to evolve, shaping the digital landscape with innovations and capabilities that push the boundaries of web development. The JavaScript Online Course has been designed to help aspiring professionals prepare for a career in this field. Moreover, these training programs help one prepare for various interview questions.

Top 40 JavaScript Interview Questions and Answers

Here is a collection of 40 important JavaScript interview questions that cover various fundamental and advanced concepts.

Questions On Basics and Fundamentals

1. What is JavaScript and its key features?

JavaScript is a dynamic programming language used for web development. Its key features include versatility in client-side scripting, asynchronous operations, and manipulation of HTML/CSS for interactive web experiences.

2. How do you comment in JavaScript?

In JavaScript, single-line comments start with //, while multi-line comments use /* */ to enclose the comment text, allowing developers to annotate code for readability and explanation.

3. Explain the differences between “var”, “let”, and “const”

“var” is function-scoped, while “let” and “const” are block-scoped. “let” allows variable re-assignment but not re-declaration in the same scope. “const” requires initialization during declaration and maintains a constant, immutable value. Unlike “var”, both “let” and “const” support block-level scoping in JavaScript.

4. Mention the different data types in JavaScript?

JavaScript has six primitive data types: “string”, “number”, “boolean”, “null”, “undefined”, and “symbol”, along with the complex data type “object”.

5. How does JavaScript handle asynchronous operations?

JavaScript uses asynchronous operations via callbacks, promises, and async/await. Callbacks execute functions once an operation is completed. Promises and async/await provide more structured and readable ways to handle asynchronous tasks.

6. Explain the concept of type coercion in JavaScript

Type coercion in JavaScript is the automatic conversion of one data type to another during operations. It occurs when operands of different types are involved in an operation.

7. What are truthy and falsy values in JavaScript?

Truthy values in JavaScript are those that evaluate to “true” in a Boolean context, while falsy values evaluate to “false”. Examples include empty strings, 0, null, undefined, and false.

8. How do you compare two objects in JavaScript?

In JavaScript, objects are compared by reference, not by their contents. To compare their contents, one needs to iterate through their properties and compare them individually.

9. Explain the concept of scope in JavaScript

Scope in JavaScript refers to the accessibility and visibility of variables. It determines where in the code a variable can be accessed, based on its declared location—global or local to a function/block.

10. What is the purpose of “strict mode” in JavaScript?

Strict mode in JavaScript enables a stricter set of rules, catching common coding mistakes, preventing the use of undeclared variables, and throwing more exceptions for silent errors.

Questions On Functions and Closures

11. What are arrow functions in JavaScript?

Arrow functions in JavaScript are concise function syntax introduced in ES6. They provide a more compact way to write functions, using “=>“ syntax and maintaining a lexical “this” context.

12. Explain the concept of higher-order functions

Higher-order functions in JavaScript are functions that can accept other functions as arguments or return functions. They enable abstraction, code reusability, and functional programming paradigms.

13. What is the difference between function declaration and function expression?

Function declaration is a named function defined using the function keyword, hoisted to the top of the scope.

Function expressions, however, are anonymous functions assigned to a variable, not hoisted, and can be defined dynamically at runtime within other expressions or statements.

14. What is a closure and how does it work in JavaScript?

A closure in JavaScript is a combination of a function and the lexical environment in which it was declared. It allows a function to access its outer scope's variables even after execution.

15. How do you create a self-invoking function in JavaScript?

To create a self-invoking function in JavaScript, enclose the function declaration in parentheses and immediately invoke it by adding another pair of parentheses at the end: “(function() { /* code */ })();”

Questions On Arrays and Objects

16. How do you iterate through an object in JavaScript?

In JavaScript, you can iterate through an object using a “for...in” loop or “Object.keys()”, “Object.values()”, or “Object.entries()” methods to access its keys, values, or key-value pairs, respectively.

17. Explain JavaScript’s “map()”, “filter()”, and “reduce()” methods.

The “map()” method transforms each element of an array, “filter()” creates a new array based on a condition, and “reduce()” reduces an array to a single value by applying a function.

18. What is object destructuring in JavaScript?

Object destructuring in JavaScript allows for extracting multiple properties from an object and assigning them to variables using a concise syntax like “{ prop1, prop2 } = object”.

19. How do you clone an object in JavaScript?

In JavaScript, you can clone an object using methods like “Object.assign({}, originalObject)” or the spread operator “{ ...originalObject }”, creating a shallow copy of the object.

20. Explain the concept of prototypal inheritance in JavaScript

Prototypal inheritance in JavaScript allows objects to inherit properties and methods from other objects through their prototype chain, providing a flexible and dynamic way of sharing functionality between objects.

Questions On DOM Manipulation

21. How do you access and modify DOM elements using JavaScript?

You can access and modify DOM elements in JavaScript using methods like “getElementById()”, “querySelector()”, and then manipulate their properties or content using JavaScript commands and properties.

22. What is event delegation in JavaScript?

Event delegation in JavaScript is a technique where a single event handler is attached to a parent element to manage events for multiple child elements, improving performance and reducing memory usage.

23. Explain the concept of bubbling and capturing in event handling

Bubbling and capturing are two phases in event propagation. Bubbling is when an event triggers the innermost element and propagates outward to its ancestors. Capturing is the opposite.

24. How do you create, append, and remove HTML elements dynamically?

To create HTML elements dynamically in JavaScript, use “document.createElement()”, then append them to the DOM using “appendChild()” and remove them using “removeChild()” or “parentNode.removeChild()”.

Questions On Error Handling and Debugging

25. How do you handle errors in JavaScript?

In JavaScript, errors can be handled using try...catch blocks to encapsulate potentially problematic code and gracefully manage exceptions by executing fallback actions or displaying error messages.

26. What are some common debugging techniques in JavaScript?

Common debugging techniques in JavaScript include using console.log(), breakpoints in browser developer tools, utilizing the debugger keyword, and employing try...catch blocks to identify and resolve issues.

27. Explain “try”, “catch”, and “finally” in JavaScript error handling

“try” allows testing a block of code for errors, "catch" catches exceptions and handles errors, and "finally" executes code, regardless of the result, ensuring cleanup.

Questions On Asynchronous JavaScript

28. What are promises in JavaScript?

Promises are objects representing the eventual completion or failure of an asynchronous operation, allowing handling of asynchronous tasks more cleanly using chaining and avoiding callback hell.

29. How do you use “async” and “await” in JavaScript?

You mark a function as “async” to enable the use of “await” inside it, allowing asynchronous code to be written in a more synchronous-like manner for cleaner, easier-to-read code.

30. Explain the difference between “setTimeout()” and “setInterval()”

“setTimeout()”  executes a function once after a specified delay, while “setInterval()” repeatedly executes a function at defined intervals until stopped, using a specified delay between each execution.

Questions On ES6 and Beyond

31. What are template literals in JavaScript?

Template literals are strings allowing embedded expressions and multiline syntax using backticks (“), enabling interpolation of variables and expressions directly within the string for more flexible and readable code.

32. Explain the concept of modules in ES

ES6 introduced modules, enabling the organization of JavaScript code into separate files, each containing its own scoped variables, functions, or classes, allowing better code organization and reuse.

33. What is the spread operator and how is it used in JavaScript?

The spread operator (…) expands an iterable (like an array) into individual elements. It's used in array literals, function arguments, or with objects to clone or merge values.

34. What are generators in JavaScript?

Generators in JavaScript are functions that can be paused and resumed, allowing iterative-like behaviour. They produce a sequence of values lazily, enhancing control flow and enabling asynchronous operations.

Miscellaneous Questions

35. What is JSON and how is it different from JavaScript objects?

JSON (JavaScript Object Notation) is a lightweight data interchange format. It resembles JavaScript objects but is a text format for data exchange, commonly used in web services.

36. How do you handle cross-origin requests in JavaScript?

Cross-origin requests in JavaScript are managed using CORS (Cross-Origin Resource Sharing), allowing controlled access to resources from different origins by configuring server headers to permit or restrict access.

37. Explain the concept of event loop in JavaScript

The event loop manages the execution of code in JavaScript, handling asynchronous tasks by queuing them in a loop, ensuring non-blocking behaviour and managing callbacks and I/O operations efficiently.

38. What is the difference between “==“ and “===“ in JavaScript?

“==“ checks for equality after performing type coercion, allowing different types to be compared by converting them.

“===“ checks for strict equality without type coercion, ensuring both value and type match.

39. How do you convert a string to a number in JavaScript?

You can convert a string to a number in JavaScript using functions like “parseInt()” or “parseFloat()”, or by using the “Number()” constructor.

40. What are Web Workers in JavaScript?

Web Workers are scripts that run in the background, separate from the main JavaScript execution thread, enabling parallel processing to handle intensive tasks without blocking the user interface.

These questions cover a wide range of topics in JavaScript, and help aspiring professionals prepare well for their interviews.

Conclusion

In summary, JavaScript's pervasive influence on web development remains unparalleled. Its adaptability and extensive capabilities have propelled the evolution of user interfaces and web applications, revolutionizing the digital experience. As JavaScript continues to expand its horizons, embracing new paradigms and frameworks, its significance in creating responsive, feature-rich websites persists. One can prepare for the javascript interview questions through online training courses to boost their career. The language's versatility, coupled with a robust community, ensures its continued dominance as a cornerstone of web development. With its agility to adapt to changing technologies, JavaScript stands poised to shape the future of the digital realm, driving innovation and redefining the possibilities of web-based interactions.