JavaScript Interview Questions:-

1. What is the purpose of using the debugger statement in JavaScript?

2. How do the == and === operators differ in JavaScript?

3. What is the distinction between var and let in JavaScript?

4. Can you describe implicit type coercion in JavaScript?

5. Is JavaScript a statically typed or dynamically typed language? Explain.

6. What does the NaN property represent in JavaScript?

7. What is the difference between passing arguments by value and by reference?

8. What is an Immediately Invoked Function Expression (IIFE) in JavaScript?

9. What is strict mode in JavaScript, and what are its key characteristics?

10. Can you explain the concept of Higher-Order Functions in JavaScript?

11. How does the this keyword work in JavaScript?

12. What are Self-Executing Anonymous Functions in JavaScript?

13. Explain the purpose and differences between call(), apply(), and bind() methods.

14. How do the exec() and test() methods function in JavaScript?

15. What is function currying, and how is it useful in JavaScript?

16. What are the benefits of using external JavaScript files in a project?

17. Can you describe the scope and scope chain mechanism in JavaScript?

18. What are closures in JavaScript, and how do they work?

19. What are some key benefits of using JavaScript for web development?

20. What is the role of object prototypes in JavaScript?

21. How do callbacks work in JavaScript?

22. What are the different types of errors in JavaScript?

23. What is memoization, and how does it improve JavaScript performance?

24. Can you explain recursion and its use in JavaScript?

25. What is the significance of a constructor function in JavaScript?

26. What is the DOM (Document Object Model) in JavaScript?

27. Which method is used to access a character at a specific position in a string?

28. What is the BOM (Browser Object Model) in JavaScript?

29. How do client-side JavaScript and server-side JavaScript differ?

30. How do null and undefined differ in JavaScript?

31. What are arrow functions in JavaScript, and how do they differ from regular functions?

32. Can you explain the prototype design pattern with an example?

33. How does the rest parameter work in JavaScript, and when should you use it?

34. How many different ways can you create an object in JavaScript?

35. What is the purpose of promises in JavaScript, and how do they improve asynchronous code?

36. How do classes work in JavaScript, and how do they compare to traditional constructor functions?

37. What are generator functions in JavaScript, and how do they differ from regular functions?

38. Can you explain WeakSet in JavaScript and how it differs from a regular Set?

39. Why are callbacks used in JavaScript, and what are the potential drawbacks?

40. What is the difference between prototypal and classical inheritance in JavaScript?

41. What is the Temporal Dead Zone in JavaScript, and how does it affect variable declarations?

42. How would you define JavaScript Design Patterns, and why are they important?

43. Does JavaScript use pass-by-value or pass-by-reference, and how does it impact variable assignments?

44. How do Async/Await and Generators differ in handling asynchronous operations?

45. What are the different primitive data types in JavaScript, and how are they stored in memory?

46. What is the significance of deferred scripts in JavaScript, and when should they be used?

47. How can you implement Lexical Scoping effectively in JavaScript?

Top 10 JavaScript Interview Questions

1. What is a Closure in JavaScript ?

Answer:-

A closure in JavaScript is a combination of a function and the scope in which it was created. This means that even after the outer function has finished executing, the inner function still has access to the variables from its original scope. Closures enable useful programming techniques such as data encapsulation, currying, and handling asynchronous operations.

Closure in JavaScript

Example 1: Understanding Closures

var number1 = 2;

var add = function () {
  var number2 = 3;

  return function () {
    return number1 + number2;
  };
};

var addNum = add();

console.dir(addNum);

In this example, addNum is a function that retains access to both number1 and number2, even though add() has already completed execution. This behavior occurs due to JavaScript's scope mechanism:

Global Scope: The variable number1 is declared outside of any function, making it globally accessible. Any function can reference number1 because it exists at the top level. Closure Scope: The variable number2 is defined inside the add() function. When add() returns the inner function, that inner function retains a reference to number2 because of closures. Even though add() has finished running, the returned function still "remembers" number2 due to this preserved scope. As a result, calling addNum() successfully retrieves and adds number1 from the global scope and number2 from the closure scope.

The console.dir(addNum); statement outputs a detailed breakdown of the function’s properties, including its closure, making it easier to inspect how JavaScript maintains references to variables.

2. Can you explain the concept of Pure Function in JavaScript?

Answer:

Pure functions play a crucial role in functional programming because they are predictable, making them simpler to understand, debug, and test. To qualify as pure, a function must adhere to two key principles:

Determinism:

– A pure function consistently produces the same output for a given input, ensuring reliability.

No Side Effects:

– The function should not cause any observable changes beyond returning a value. It must not alter application state outside its scope.

Non-Deterministic Functions

Certain functions are considered non-deterministic because their output can vary even with identical inputs. Examples include functions that depend on:

Random number generation.

A global variable that may change.

A parameter that could be modified elsewhere.

The system clock or current time.

Side Effects in Functions

A function exhibits side effects when it modifies external state in ways beyond returning a value. Common examples include:

Altering a global variable or an object property.

Printing messages to the console.

Writing data to a file, screen, or network.

Throwing an error instead of returning an error-related result.

Initiating any external process.

Why Redux Reducers Must Be Pure

Redux enforces purity in reducer functions to maintain predictable state management. If a reducer contains side effects or behaves non-deterministically, the application's state can become inconsistent, leading to unpredictable behavior. Furthermore, impure reducers disrupt advanced debugging features like time-travel debugging and may cause React components to render stale or incorrect data. Ensuring reducers remain pure leads to a more stable and maintainable application.

3. Explain what is Function Composition in JavaScript?

Answer:-

Function composition refers to the technique of combining multiple functions to create a new function or execute a computation. It follows this structure:

(f∘g)(x)=f(g(x))

This means that function 𝑔 g is applied to 𝑥 x first, and then function 𝑓 f is applied to the result. In JavaScript, function composition can be implemented as follows:

const compose = (f, g) => (x) => f(g(x));

const g = (num) => num + 1;
const f = (num) => num * 2;

const h = compose(f, g);

console.log(h(20)); // 42

Here, g first increments 20 to 21, and then f doubles it to produce 42.

Function Composition in React

In React development, function composition is useful for organizing complex component structures. Instead of deeply nesting components, developers can compose them into higher-order components (HOCs). This approach allows for modular, reusable logic that enhances any component by wrapping it with additional functionality, leading to cleaner and more maintainable code.

4. Explain what is Functional Programming?

Answer:-

Functional programming is a paradigm that centers around pure functions as its building blocks. In software development, composition plays a crucial role, and different programming paradigms are often named based on their primary unit of composition:

Object-Oriented Programming (OOP): Objects serve as the fundamental unit of composition.

Procedural Programming: Procedures (or routines) act as the core units of composition.

Functional Programming: Functions are the main units of composition.

Being a declarative paradigm, functional programming focuses on describing what a program should accomplish rather than specifying how to achieve it. This approach enhances code readability, simplifies debugging, and improves maintainability compared to imperative programming. Additionally, functional programs tend to be more concise, reducing overall complexity.

Key Principles of Functional Programming

Immutability – Data structures remain unchanged, making the program easier to reason about and reducing unintended side effects.

Higher-Order Functions – Functions that accept other functions as parameters or return functions as results, enabling greater modularity and reusability.

Avoiding Shared Mutable State – Sharing mutable state leads to unpredictable behavior, making the program harder to debug and test. By avoiding it, functional programming ensures better predictability and correctness.

5. Define the concept of promise in JavaScript ?

Answer:-

A Promise in JavaScript is an object that represents the eventual completion (or failure) of an asynchronous task. It serves as a placeholder for a value that is not immediately available because the operation is still in progress.

Key Features of Promises State Management – A Promise can be in one of three states:

Pending – The initial state, where the outcome is still unknown.

Fulfilled – The operation completed successfully, resolving with a value.

Rejected – The operation failed, returning an error.

Immutability – Once a Promise settles (either fulfilled or rejected), its state is final and cannot be changed. This makes it a predictable and reliable tool for handling asynchronous operations.

Chaining – Promises support method chaining, allowing the result of one Promise to feed into another. This is done using .then() for handling successful outcomes and .catch() for handling errors, making asynchronous workflows more structured and readable—similar to function composition in functional programming.

Example: Using a Promise

const promise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("Operation Successful!");
    // Or reject(new Error("Something went wrong!")) in case of failure.
  }, 1000);
});

promise
  .then((result) => {
    console.log(result); // "Operation Successful!"
  })
  .catch((error) => {
    console.error(error);
  });

Simplifying Promises with Async/Await

JavaScript provides async/await syntax, which makes working with Promises feel more like synchronous code, improving readability and maintainability.

const processData = async () => {
  try {
    const data = await fetchData(); // Waits for the Promise to resolve
    console.log("Processed Data:", data);
  } catch (error) {
    console.error("Error:", error); // Handles rejection
  }
};

6. Explain what is TypeScript?

Answer:-

TypeScript is an open-source superset of JavaScript, developed and maintained by Microsoft. It has gained widespread adoption in recent years, and if you work with JavaScript, chances are you'll encounter TypeScript at some point. Its primary advantage is the introduction of static typing to JavaScript, which is inherently dynamically typed. By enforcing type safety, TypeScript helps developers catch errors early, leading to better code quality and easier maintenance.

Key Features of TypeScript Static Typing – Variables, function parameters, and return values can have explicitly defined types, reducing runtime errors and ensuring consistency in the codebase.

Enhanced IDE Support – TypeScript enhances developer productivity with better autocompletion, navigation, and refactoring tools in modern IDEs.

Compilation Process – TypeScript code is transpiled into JavaScript, making it runnable in any JavaScript environment. This compilation phase catches type-related errors before execution.

Interfaces – Interfaces define contracts for objects and functions, ensuring a consistent structure across your code.

Seamless JavaScript Compatibility – TypeScript works well with existing JavaScript code, allowing for gradual adoption in projects without requiring a full rewrite.

Example: Defining Interfaces and Types

interface User {
  id: number;
  name: string;
}

type GetUser = (userId: number) => User;

const getUser: GetUser = (userId) => {
  return {
    id: userId,
    name: "John Doe",
  };
};

TypeScript and Code Quality

While TypeScript helps prevent type-related issues, it is not a replacement for best practices like code reviews, Test-Driven Development (TDD), and linting tools (e.g., ESLint). Type correctness does not equate to program correctness, but TypeScript can occasionally catch errors that slip through other quality control measures. Its greatest advantage lies in improving the developer experience, streamlining code navigation, and reducing debugging time.

7. What is Web Component in JavaScript?

Answer:-

Web Components are a collection of native browser APIs that enable developers to create custom, reusable, and encapsulated HTML elements. These components integrate seamlessly with web pages and applications without requiring external frameworks or libraries, as they rely solely on standard HTML, CSS, and JavaScript.

One of the biggest advantages of Web Components is their framework-agnostic nature, making them ideal for large teams working across different front-end technologies. Companies like Adobe use Web Components in their Spectrum design system, ensuring smooth integration with frameworks like React, Angular, and Vue.

Although Web Components have been around for years, their adoption has surged recently, especially in enterprise environments. They are now a W3C standard and are fully supported by all major browsers.

Example: Creating a Simple Web Component

The following example demonstrates how to define and use a basic Web Component:

<script>
  // Define a new custom HTML element
  class SimpleGreeting extends HTMLElement {
    constructor() {
      super();
      const shadowRoot = this.attachShadow({ mode: "open" });
      shadowRoot.innerHTML = `
        <style>
          p {
            font-family: Arial, sans-serif;
            color: var(--color, black); /* Uses a CSS variable */
          }
        </style>
        <p><slot>Hello, Web Components!</slot></p>
      `;
    }

    static get observedAttributes() {
      return ["color"]; // Observing changes to the "color" attribute
    }

    attributeChangedCallback(name, oldValue, newValue) {
      if (name === "color") {
        this.style.setProperty("--color", newValue);
      }
    }
  }

  // Register the custom element
  customElements.define("simple-greeting", SimpleGreeting);
</script>

<!-- Using the Web Component -->
<simple-greeting>Hello, reader!</simple-greeting>
<simple-greeting color="blue">Hello, World!</simple-greeting>

Key Features of Web Components

Encapsulation – Styles and markup are contained within a shadow DOM, preventing conflicts with the rest of the page.

Reusability – Once defined, a Web Component can be used anywhere, promoting modular development.

Framework Independence – Works in any framework or vanilla JavaScript without additional dependencies.

Custom Attributes & Slots – Components can accept attributes (e.g., color) and slots to customize content dynamically.

8. How do you explain React Hook?

Answer:-

React Hooks are special functions that enable you to use state and other React features without needing to write class components. They allow you to manage state, context, refs, and lifecycle events in functional components, providing a more flexible way to structure your code. With Hooks, you can encapsulate related logic within a single function while keeping unrelated logic separate, leading to better code organization and readability.

Essential React Hooks

useState – Introduces state in functional components, preserving values across re-renders.

useEffect – Handles side effects (such as fetching data or subscribing to events). It consolidates behaviors that were previously spread across componentDidMount, componentDidUpdate, and componentWillUnmount.

useContext – Provides a way to consume context in function components without needing higher-order components or render props.

useRef – Creates a mutable reference that persists across re-renders, commonly used for accessing DOM elements or storing instance values.

Custom Hooks – Allow developers to encapsulate and reuse logic across multiple components without modifying the component hierarchy.

Rules of Hooks

React enforces a few key rules to ensure Hooks work correctly:

✅ Use Hooks at the top level – Avoid calling them inside loops, conditions, or nested functions.

✅ Use Hooks only in React function components or custom Hooks – They should not be used in regular JavaScript functions.

Why Hooks?

Hooks solve several issues that were common in class components:

Eliminating the need to bind methods in the constructor.

Reducing complexity by grouping related logic together.

Making it easier to share and reuse stateful logic without altering the component structure.

9. How Do you Create a Click Counter in React?

Answer:-

In React, you can create a simple click counter using the useState Hook. This allows you to manage component state without needing a class component. Here's how:

import React, { useState } from "react";

const ClickCounter = () => {
  const [count, setCount] = useState(0); // State variable to track clicks

  return (
    <div>
      <p>You have clicked {count} times</p>
      <button onClick={() => setCount((prevCount) => prevCount + 1)}>
        Click me
      </button>
    </div>
  );
};

export default ClickCounter;

Why Use a Function in setCount?

When updating state based on its previous value, it's recommended to pass a function to setCount. This ensures that React always works with the most recent state, avoiding potential issues with stale state updates in asynchronous operations.

10. What is Test Driven Development (TDD)?

Answer:-

Test-Driven Development (TDD): Writing Tests Before Code

Test-Driven Development (TDD) is a software development methodology where tests are written before the actual implementation. This iterative process ensures that code meets predefined requirements while minimizing bugs. TDD fosters higher code quality, better design, and increased developer productivity by maintaining a disciplined feedback loop.

Why TDD Matters

One of the biggest roadblocks to continuous deployment is the fear of change—developers hesitate to modify code due to potential unintended side effects. TDD alleviates this fear by ensuring that the code is always deployable, making it easier to release new features and bug fixes frequently and confidently.

Advantages of Writing Tests First

✅ Better Code Coverage – When tests are written first, they are more likely to cover all edge cases, rather than being an afterthought.

✅ Improved API Design – Writing tests upfront forces developers to think critically about API design, preventing unnecessary complexity or leaky abstractions.

✅ Fewer Bugs – Bugs are caught early in development, making them easier and cheaper to fix.

✅ Cleaner, More Maintainable Code – TDD encourages writing modular, loosely coupled code, which leads to better long-term maintainability.

The TDD Workflow

1️⃣ Write a Test – Define a test for the intended functionality. Since no implementation exists yet, the test will fail initially.

2️⃣ Write Just Enough Code – Implement the minimal code needed to pass the test.

3️⃣ Refactor with Confidence – Once the test passes, refactor the code to improve structure and maintainability while ensuring that the test still passes.

4️⃣ Repeat – Iterate this cycle for each requirement, gradually building robust software while keeping all tests green.

Challenges of TDD

🔹 Learning Curve – TDD requires discipline and practice. Even after 6 months, it may feel slow and tedious. However, after 2 years, it becomes second nature—and your productivity will likely surpass those who don't practice TDD.

🔹 Perceived Time Cost – Initially, writing tests first may seem time-consuming, but it actually saves time in the long run by reducing bugs and making refactoring safer. As the saying goes:

🗣️ “If you think you don’t have time for TDD, you definitely don’t have time to skip it.”