A Complete Guide to React Coding Interview Questions

React is a popular JavaScript library developed by Facebook for building interactive and efficient user interfaces, particularly for single-page applications (SPAs). It enables developers to create dynamic web applications with minimal effort by using a component-based architecture.

What are the major features of React?

• Component-Based Architecture: React allows developers to build reusable UI components that manage their own state and compose them to create complex UIs.
• Declarative Syntax: React makes it easier to design interactive UIs by describing how the UI should look based on the application state.
• One-Way Data Binding: React provides a one-way data flow that makes the application predictable and easier to debug.
• Virtual DOM: React optimizes performance by using a virtual representation of the DOM and only updates the necessary portions of the real DOM.
• JSX (JavaScript XML): A syntax extension that looks like HTML and allows you to write HTML-like structures within JavaScript code.
• Server-Side Rendering (SSR): React can be rendered on the server, improving performance and SEO.
• One-Way Data Binding: React provides a one-way data flow that makes the application predictable and easier to debug.

JSX syntax

JSX is a syntax extension for JavaScript that looks similar to HTML but enables the use of JavaScript expressions within markup. It simplifies the creation of UI components and enhances code readability.

Example:

const element = <h1>Hello, React!</h1>;

JSX must be transpiled into JavaScript before it can be executed in the browser, typically using Babel.

Virtual DOM 

The Virtual DOM (VDOM) is a lightweight copy of the actual DOM. React updates the VDOM first and then applies the necessary changes to the real DOM, significantly improving performance by reducing unnecessary re-renders.

How It Works:

1. React creates a Virtual DOM representation of the UI.
2. When the state changes, React compares the new VDOM with the previous one (Diffing Algorithm).
3. Only the changed elements are updated in the actual DOM (Reconciliation Process), minimizing performance overhead.

server-side rendering 

Server-side rendering (SSR) allows React components to be rendered on the server and sent to the client as fully formed HTML pages before the JavaScript code is loaded.

Importance of server-side rendering for Search Engine Optimizations(SEO):

Importance for SEO:

• Improves search engine discoverability by serving fully rendered content to web crawlers.
• Enhances initial page load speed, improving user experience and reducing bounce rates.
• Helps in better indexing for pages that rely on dynamic content.

Unidirectional or one-way data flow or data binding.

React enforces a one-way data flow, meaning that data moves from the parent component to child components via props. This approach ensures a more predictable and manageable state structure, making debugging easier.

React enforces a one-way data flow, where data moves from parent components to child components via props. This approach creates a more predictable and manageable state structure, making debugging easier.

Key Benefits:

• Enhances performance by reducing unnecessary re-renders.
• Maintainability can be increased by making data flow explicit.
• Prevents unintended side effects by limiting the modification of data.

To manage complex states, React recommends using state management solutions such as Context API, Redux, or Zustand.

Reusable and Composable:

React promotes reusability through its component-based architecture. Developers can create small, independent components and compose them into more complex user interfaces.

Advantages:

• Reduces code duplication and improves maintainability.
• Enhances scalability by allowing the reuse of components across different projects.
• Encourages modular development, making debugging and testing easier.

At the beginner level, interviewers assess candidates understanding of foundational react coding interview questions. Understanding with JavaScript, JSX, components, props, and state is important.

1. What is JSX? Why is it used in React?

JSX (JavaScript XML) is a syntax extension for JavaScript that allows developers to write HTML-like code within JavaScript. It closely resembles HTML but is transpired into regular JavaScript by tools like Babel before it runs in the browser. JSX is not required to use React, but it simplifies creating and visualizing UI components.

2. Write a simple React component that displays “Hello, World!”.

React components are reusable pieces of code that define how a part of the UI should look and behave. To create a simple React component that displays "Hello, World!", follow these steps:

import React from 'react';

const HelloWorld = () => {
    return <h1>Hello, World!</h1>;
};

export default HelloWorld;

3. What is the difference between state and props in React?

In React, props and states are essential concepts for managing data in components. Props (short for "properties") pass data from a parent component to its child components.

4. What are React lifecycle methods?

Lifecycle methods are special methods in class components, such as componentDidMount, componentDidUpdate, and componentWillUnmount, which allow developers to execute code at different stages of a component’s lifecycle.

5. How does React handle events?

In React, handling events is slightly different from how it is done in traditional HTML and JavaScript. React uses synthetic events, and wrappers around the DOM events, providing consistent behavior across different browsers.

Here’s how React handles events:

• CamelCase Syntax: React uses camelCase for event names instead of lowercase. For example, on becomes onClick, and onchange becomes onChange.
• Event Handlers as Functions: Functions are used as event handlers, and they are typically passed as props to elements.
• Prevent Default Behavior: The preventDefault method can be called within the event handler to stop the default behaviour of an element. Unlike traditional JavaScript, you cannot return false to achieve this.
• Binding Context: In class components, event handlers often need to be explicitly bound to the component instance using .bind() or by using arrow functions. In functional components, this is not necessary when using hooks.

6. What are keys in React, and why are they important?

Keys in React are unique identifiers assigned to elements in a list. They are essential for React's reconciliation process, which determines how the DOM should be updated efficiently when data changes.

Example of Using Keys:

const App = () => {
    const items = ['Apple', 'Banana', 'Cherry'];

    return (
        <ul>
            {items.map((item, index) => (
                <li key={index}>{item}</li> // "key" uniquely identifies each list item
            ))}
        </ul>
    );
};

export default App;

Output:

Apple
Banana
Cherry

7. How to Conditionally Render Components in React

In react interview coding questions, conditional rendering means showing or hiding  based on certain conditions. Since JSX is JavaScript under the hood, developers can use JavaScript expressions like if-else, ternary operators, or logical AND (&&) to control what gets rendered.

Methods for Conditional Rendering

• Using Ternary Operator: The ternary operator concisely renders one of two elements based on a condition.
• Using Logical AND (&&): The logical AND operator renders something only when a condition is true.
• Using If-Else Statements: You can use traditional if-else statements, but this method may require returning elements explicitly, especially for larger conditions.

1. Design a stopwatch application that enables users to start, pause, and reset the timer. Utilize React state, event handlers, and either setTimeout or setInterval functions to control the timer's functionality and behaviour.

A stopwatch application enables users to start, pause, and reset a timer. Here’s how to implement it in React:

• State Management: Use useState to track time and running status.
• Timer Control: Use setInterval to update time while running and clearInterval to stop it.
• Event Handlers: Attach onClick events to Start, Pause, and Reset buttons.

Implementation Steps:

1. Initialize state variables (time, isRunning).
2. Use useEffect to start and stop the timer when the state changes.
3. Provide buttons for user interactions.
4. Display the elapsed time in a formatted way.

2. Create a Toggle Switch that can switch between "On" and "Off" states, clearly indicating its current status with color 

A toggle switch should visually and functionally switch between "On" and "Off" states:

• State Management: Use useState to track the toggle state.
• Event Handling: Toggle the state when clicked.
• Styling: Change color based on the state.

Implementation Steps:

1. Create a useState hook to manage the switch state.
2. Use a button or a div styled to represent the switch.
3. Change color dynamically using inline styles or class names.
4. Update the state on click.

3. Develop a To-Do List App that allows users to add, edit, and delete tasks. Include features for marking tasks as complete

A To-Do List app should allow users to add, edit, delete, and mark tasks as complete.

• State Management: Use useState to store tasks.
• Adding Tasks: Store input value and update state when submitted.
• Editing & Deleting: Provide buttons to modify or remove tasks.‍
• Marking as Complete: Toggle task status when clicked.

How to Implement:

1. Set up state for task storage.
2. Render tasks dynamically from state.
3. Implement event handlers for adding, editing, deleting, and marking tasks.
4. Use conditional styling to indicate completed tasks.

4. Build a Dropdown Menu that displays a list of items when clicked.

A dropdown menu should display a list of items when clicked:

• State Management: Use useState to track visibility.
• Event Handling: Toggle menu visibility when clicked.
• Styling & Animation: Show/hide menu with smooth transitions.

Implementation Steps:

1. Create a button to open the menu.
2. Use state to toggle the menu’s visibility.
3. Render a list of items conditionally.
4. Add click listeners for closing the menu when clicking outside.

5. What is the virtual DOM?

The Virtual DOM (VDOM) is a lightweight JavaScript representation of the actual DOM. React updates the Virtual DOM first, compares it with the previous version (diffing), and efficiently updates only the changed parts in the real DOM. Benefits include improved performance and reduced unnecessary re-renders.

6. Why use React instead of other frameworks like Angular?

React and Angular are both widely used JavaScript frameworks, but they are fundamentally different and this is partly why many developers prefer to use React. For one React uses a component-based architecture. This makes React less strictly defined and less heavy than Angular. Unlike Angular which uses a full-fledged MVC (Model-View-Controller) structure, React is primarily concerned with building UI using components. Components are reusable and allow developers to develop user interfaces with less repetition - this generally speeds up development time.

React is easier to learn than Angular, primarily because Angular requires developers to learn the TypeScript language and have a general understanding of the extensive features It includes. React is less intimidating to pick-up, especially for a developer with some JavaScript experience and knowledge of React. People also love that React is flexible. Angular is a more rigid framework and requires developers to use Angular's built-in tools and perform development using Angular's design patterns; React is much more fluid and comfortable when you want to combine libraries and frameworks seamlessly.

At the intermediate level, the candidate will be expected to show a wider range of knowledge on React, including hooks, performance tuning, and other state management libraries (e.g., Redux). Here are the react interview coding questions for intermediate level:

1. What is useEffect, and how does it work?

useEffect is a hook for handling side effects in functional components. It runs after every render by default but can be controlled using a dependency array. For example:

useEffect(() => {
    console.log("Component rendered");
}, []); // Runs only on mount

2. Write a higher-order component (HOC) that logs props.

const withLogging = (WrappedComponent) => {
    return (props) => {
        console.log(props); // Logs the props passed to the WrappedComponent
        return <WrappedComponent {...props} />; // Renders the original component with its props
    };
};

export default withLogging;

The provided code defines a higher-order component (HOC) in React. A higher-order component is a function that takes a component as its argument, enhances or modifies its behaviour, and then returns a new component. HOCs are a common pattern in React for code reuse and behaviour extension.

3. What is the Context API, and how is it different from Redux?

The Context API allows passing data through the component tree without prop-drilling. Unlike Redux, it doesn’t have middleware or built-in state management capabilities. Here are the differences between these two:

4. How do you optimize rendering performance in a React application?

Techniques include using React.memo, avoiding anonymous functions in JSX, and implementing virtualized lists for rendering large datasets.

5. What is the purpose of useReducer, and how is it different from useState?

When state transitions depend on specific actions, the useReducer hook manages complex state logic. useReducer is an alternative to useState, but it is more powerful and suitable for scenarios involving multiple state values or intricate transitions.

Unlike useState, useReducer relies on a reducer function to determine the next state. This function takes the current state and an action as its arguments and returns a new state based on the action type.

6. What are controlled and uncontrolled components?

In React, components can be classified into two main types based on how they manage their state: controlled and uncontrolled components. These terms specifically refer to how a component handles its form elements. The distinction comes down to whether the component's state is controlled by React or by the DOM itself.

Controlled Components:

A controlled component is a form element whose value is controlled by the state within the React component. In other words, React is responsible for managing the state of the input fields and updating the UI based on the state.

In a controlled component:

• The component's value is always set to the state via props.
• Any changes to the form element trigger an event that updates the component's state (usually via the onChange event handler).
• The value of the form element is always synchronized with the React state.

Uncontrolled Components:

An uncontrolled component is a form element whose value is managed by the DOM itself, not React. In this case, React does not directly manage the state of the form element, and the value is accessed through a ref (reference) to the DOM element.

In an uncontrolled component:

• The component does not keep track of the input field's state in React.
• The form element's value is stored in the DOM and is accessed through a reference (ref).
• Updates to the input field happen directly within the DOM, and React is not directly involved in the value update.

7. What is the difference between the ES6 and ES5 standards?

ES6 (ECMAScript 2015) introduced significant improvements over ES5. Here’s a quick comparison:

8. When dealing with multiple input types and controlled components, what techniques can you use to handle state management for forms in React? Additionally, how would you approach form validation, including conditional validations based on user inputs?

State Management for Forms in React

Handling state in React forms, especially with multiple input types, involves using controlled components and various state management techniques.

Techniques for Managing Form State

useState Hook: Manage individual inputs as state variables.

Example:

const [formData, setFormData] = useState({ name: "", email: "" });

const handleChange = (e) => {
  setFormData({ ...formData, [e.target.name]: e.target.value });
};

useReducer Hook: Useful for complex forms with multiple updates.

Example:

const formReducer = (state, action) => {
  return { ...state, [action.name]: action.value };
};

const [formState, dispatch] = useReducer(formReducer, { name: "", email: "" });

const handleChange = (e) => {
  dispatch({ name: e.target.name, value: e.target.value });
};

React Hook Form (Third-Party Library): Reduces re-renders and simplifies validation.

Example:

const { register, handleSubmit } = useForm();

Approaching Form Validation

Basic Validation: Use state to track errors and validate fields on submission.

Conditional Validation: Apply validation rules dynamically based on input values.

Example:

if (formData.userType === "admin" && !formData.adminCode) {
  setError("Admin code is required for admins.");
}

9. How can you effectively use comments in React to enhance code readability and maintainability, particularly when dealing with complex components or nested structures? 

Writing clear and concise comments in React is essential for improving code readability, especially when dealing with complex components or deeply nested structures. Comments help developers understand the logic behind specific code sections, making it easier to debug, update, and collaborate on projects.

Clarifying Component Purpose

At the beginning of a React component file, it is helpful to include a comment that briefly explains the component's purpose. This allows anyone reading the code to immediately understand its function without having to analyze every line.

Example:

// UserProfile.js
// This component displays user profile information, including name and email.
const UserProfile = ({ name, email }) => {
  return (
    <div>
      <h2>{name}</h2>
      <p>{email}</p>
    </div>
  );
};

export default UserProfile;

Explaining Key Functions and Handlers

When working with event handlers or utility functions within a component, comments can help clarify their role. A function name might give a hint about what it does, but a short comment explaining the logic can make it even more understandable.

Example:

// This function handles form submission by preventing default behavior
// and calling the submitForm function to process data.
const handleFormSubmit = (e) => {
  e.preventDefault();
  submitForm();
};

Breaking Down Complex JSX Structures

In cases where a component contains deeply nested JSX elements, comments can be used to indicate different sections. This is particularly useful in UI-heavy components that include multiple divs, conditional rendering, or mapped lists.

Example:

return (
  <div>
    {/* Header Section */}
    <header>
      <h1>Dashboard</h1>
    </header>

    {/* Sidebar Navigation */}
    <aside>
      <nav>
        <ul>
          <li>Home</li>
          <li>Profile</li>
          <li>Settings</li>
        </ul>
      </nav>
    </aside>

    {/* Main Content */}
    <main>
      <h2>Welcome to your dashboard</h2>
      <p>Here is an overview of your account.</p>
    </main>
  </div>
);

Using Comments for Conditional Rendering

React applications often include conditional rendering, which can sometimes be tricky to follow. In such cases, comments can clarify the logic behind why certain elements are displayed or hidden.

Example:

return (
  <div>
    <h1>Welcome, {user.name}</h1>

    {/* Show admin controls only if the user has admin privileges */}
    {user.isAdmin && (
      <button>Admin Dashboard</button>
    )}

    {/* Show login button if the user is not authenticated */}
    {!user.isAuthenticated && (
      <button>Login</button>
    )}
  </div>
);

10. What are some best practices for commenting in JSX versus traditional JavaScript, and how can comments be used to document component props and state management?

Commenting in JSX vs Traditional JavaScript

Commenting in JSX differs from traditional JavaScript because JSX is a combination of HTML and JavaScript. While JavaScript uses // for single-line comments and /* */ for multi-line comments, JSX requires a different approach within the return statement.

Commenting in Traditional JavaScript: In standard JavaScript, comments can be placed anywhere in the code using // for single lines and /* */ for multi-line comments.

// This function calculates the sum of two numbers
function add(a, b) {
  return a + b;
}

/* This function checks if a number is even
   and returns true or false */
function isEven(num) {
  return num % 2 === 0;
}

Commenting in JSX: In JSX, curly braces {} are used to wrap comments inside the return statement because JSX syntax closely resembles HTML.

return (
  <div>
    {/* This is a comment inside JSX */}
    <h1>Welcome to my app</h1>

    {/* Render button only if user is logged in */}
    {isLoggedIn && <button>Logout</button>}
  </div>
);

Using Comments to Document Component Props

When working with reusable components, documenting props is important for maintainability. The best approach is to use JSDoc comments above the component definition.

Example: Documenting Props in a Component

/**
 * Renders a user profile card.
 * @param {string} name - The name of the user.
 * @param {string} email - The user's email address.
 */
const UserProfile = ({ name, email }) => {
  return (
    <div>
      <h2>{name}</h2>
      <p>{email}</p>
    </div>
  );
};

Commenting on State Management in React

When dealing with useState or useReducer, adding comments helps describe what the state variables represent and how they are updated.

Example: Commenting on useState Hooks

import { useState } from "react";

const Counter = () => {
  // Stores the current count value
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
};

Example: Commenting on useReducer Hooks

For complex state logic, useReducer is often used, and adding comments helps explain the actions and reducer logic.

import { useReducer } from "react";

// Reducer function to manage counter state
const counterReducer = (state, action) => {
  switch (action.type) {
    case "increment":
      return { count: state.count + 1 };
    case "decrement":
      return { count: state.count - 1 };
    default:
      return state;
  }
};

const Counter = () => {
  // useReducer for managing counter state
  const [state, dispatch] = useReducer(counterReducer, { count: 0 });

  return (
    <div>
      <p>Count: {state.count}</p>
      <button onClick={() => dispatch({ type: "increment" })}>+</button>
      <button onClick={() => dispatch({ type: "decrement" })}>-</button>
    </div>
  );
};

11. Design a component that utilizes the useContext hook in React.

The useContext hook in React provides a way to share values like theme, authentication status, or user settings across multiple components without having to pass props manually at every level.

Example: Theme Context with useContext

import React, { createContext, useContext, useState } from "react";

// Create a Theme Context
const ThemeContext = createContext();

// Theme Provider Component
const ThemeProvider = ({ children }) => {
  const [theme, setTheme] = useState("light");

  // Function to toggle theme
  const toggleTheme = () => {
    setTheme((prevTheme) => (prevTheme === "light" ? "dark" : "light"));
  };

  return (
    <ThemeContext.Provider value={{ theme, toggleTheme }}>
      {children}
    </ThemeContext.Provider>
  );
};

// Component that uses Theme Context
const ThemedComponent = () => {
  const { theme, toggleTheme } = useContext(ThemeContext);

  return (
    <div style={{ background: theme === "light" ? "#fff" : "#333", color: theme === "light" ? "#000" : "#fff", padding: "20px" }}>
      <h2>Current Theme: {theme}</h2>
      <button onClick={toggleTheme}>Toggle Theme</button>
    </div>
  );
};

// App Component
const App = () => {
  return (
    <ThemeProvider>
      <ThemedComponent />
    </ThemeProvider>
  );
};

export default App;

In this example, the useContext hook allows ThemedComponent to access and modify the theme state without passing props down manually.

12. How Can you create a component that utilizes the useReducer hook in React?

The useReducer hook is useful for managing complex state logic, especially when multiple state transitions depend on previous state values.

Example: Counter Component with useReducer

import React, { useReducer } from "react";

// Reducer function to manage counter state
const counterReducer = (state, action) => {
  switch (action.type) {
    case "increment":
      return { count: state.count + 1 };
    case "decrement":
      return { count: state.count - 1 };
    case "reset":
      return { count: 0 };
    default:
      return state;
  }
};

const Counter = () => {
  // useReducer for managing counter state
  const [state, dispatch] = useReducer(counterReducer, { count: 0 });

  return (
    <div>
      <h2>Count: {state.count}</h2>
      <button onClick={() => dispatch({ type: "increment" })}>+</button>
      <button onClick={() => dispatch({ type: "decrement" })}>-</button>
      <button onClick={() => dispatch({ type: "reset" })}>Reset</button>
    </div>
  );
};

export default Counter;

Here, useReducer is used to manage a counter's state with actions to increment, decrement, and reset its value.

13. What is useLayoutEffect and What is the purpose of the `useLayoutEffect` Hook?

useLayoutEffect is a React hook similar to useEffect, but it runs synchronously after all DOM mutations and before the browser paints the screen. This makes it useful for measuring layout changes and applying visual updates before the browser repaints.

Purpose of useLayoutEffect

• Ensures that the DOM is updated before the screen is painted.
• Useful for synchronously measuring and updating layout-based calculations.
• Prevents flickering in UI updates that depend on layout measurements.

14. What are the differences between useEffect` and useLayoutEffect?

At the expert level, candidates must showcase their ability to design and implement complex architectures, handle testing, and combine advanced features like server-side rendering (SSR) or TypeScript. Here are React Coding Interview Questions for the expert level:

1. How do you implement SSR with React?

Server-Side Rendering (SSR) renders React components on the server, generates HTML, and sends it to the client. This can improve initial load performance and SEO, as the browser receives fully rendered HTML rather than an empty page with JavaScript.

You can implement SSR in React using frameworks like Next.js (a popular React framework for SSR) or manually rendering components with Node.js and Express.

Implementing SSR with Next.js:

Next.js is a React framework that provides built-in SSR capabilities with minimal setup. It automatically handles server-side rendering for your components based on how the page is requested.

Steps to Set Up SSR in Next.js:

Install Next.js:

npx create-next-app my-ssr-app
cd my-ssr-app

Create a Page with SSR: In Next.js, you can use the getServerSideProps function to render the page on the server for each request.

Example: pages/index.js

import React from 'react';

const HomePage = ({ message }) => {
    return (
        <div>
            <h1>Server-Side Rendered Page</h1>
            <p>{message}</p>
        </div>
    );
};

// Fetch data on the server before rendering the page
export async function getServerSideProps() {
    // Simulating fetching data from an API or database
    const message = "This content is rendered on the server!";
    return {
        props: { message }, // Will be passed to the page component as props
    };
}

export default HomePage;

Run the Next.js App:

npm run dev

In this setup, Next.js automatically handles server-side rendering, and getServerSideProps ensures that the data is fetched and the page is rendered on the server each time the page is requested.

2. Write a custom hook for data fetching.

A custom hook in React allows you to extract logic for data fetching into a reusable function. It can handle fetching, loading states, and errors, providing a cleaner and more modular approach to data management in components.

const useFetch = (url) => {
    const [data, setData] = useState(null);
    const [loading, setLoading] = useState(true);

    useEffect(() => {
        async function fetchData() {
            const response = await fetch(url);
            const result = await response.json();
            setData(result);
            setLoading(false);
        }
        fetchData();
    }, [url]);

    return { data, loading };
};

Before Fetching: When the component using this hook is rendered for the first time, the loading state is true (since it's initialized to true), and the data state is null.

After Fetching: The fetchData function is called inside the useEffect, which fetches data from the given url. Once the data is successfully fetched and converted to JSON, it sets the data state and changes loading to false.

After the data is fetched, the component will re-render, and the loading state will be false, showing the data in the component.

3. How would you integrate TypeScript into a React project?

To integrate TypeScript into a React project, first install the necessary TypeScript dependencies. This can be done using npm or yarn by running the command: npm install typescript @types/react @types/react-dom. Once the dependencies are in place, you must  rename your .js files to .tsx for components and .ts for non-JSX files. This allows TypeScript to type-check your code.

Gradually introduce type definitions by adding types for props, state, and component methods. This process helps ensure type safety and improves development experience with features like autocompletion, error detection, and refactoring support. 

4. Explain how to prevent re-renders in React.

Preventing unnecessary re-renders in React improves performance, especially in large applications. To optimize re-renders, use React.memo for functional components to memoize them, preventing re-renders unless props change. In class components, you can use shouldComponentUpdate to control when a component should update based on changes to props or state.

Additionally, be mindful of unnecessary state updates. Avoid updating state when the new state is the same as the previous one, and use state hooks like useCallback and useMemo to memoize functions and values.

5. What are some best practices for securing React applications?

Securing React applications involves implementing a combination of strategies to protect against common web vulnerabilities. Start by validating all user inputs and escaping output data to prevent injection attacks, such as XSS (Cross-Site Scripting). Use HTTPS to encrypt communication between the client and the server, ensuring that sensitive data remains protected during transmission.

6. What is Flux architecture, and how is it related to React? Provide an example of using Flux in a React app.

Flux is an application architecture introduced by Facebook for managing unidirectional data flow in React applications. It is not a library but a design pattern that helps manage state efficiently, especially in large applications.

Key Concepts of Flux:

1. Action – Describes an event that triggers a state change.
2. Dispatcher – Central hub that dispatches actions to stores.
3. Store – Holds the application state and updates in response to actions.
4. View (React Components) – Subscribes to store changes and re-renders when the store updates.

Example: Implementing Flux in a React App

1. Defining Actions

// actions.js
export const ActionTypes = {
  ADD_TODO: "ADD_TODO",
};

export const addTodo = (task) => ({
  type: ActionTypes.ADD_TODO,
  payload: task,
});

2. Creating the Dispatcher

// dispatcher.js
import { Dispatcher } from "flux";

const AppDispatcher = new Dispatcher();
export default AppDispatcher;

3. Implementing the Store

// store.js
import { EventEmitter } from "events";
import AppDispatcher from "./dispatcher";
import { ActionTypes } from "./actions";

class TodoStore extends EventEmitter {
  constructor() {
    super();
    this.todos = [];
    
    AppDispatcher.register((action) => {
      switch (action.type) {
        case ActionTypes.ADD_TODO:
          this.todos.push(action.payload);
          this.emit("change");
          break;
        default:
          break;
      }
    });
  }

  getTodos() {
    return this.todos;
  }
}

export default new TodoStore();

4. Using Flux in a React Component

// TodoApp.js
import React, { useState, useEffect } from "react";
import AppDispatcher from "./dispatcher";
import { addTodo } from "./actions";
import TodoStore from "./store";

const TodoApp = () => {
  const [todos, setTodos] = useState(TodoStore.getTodos());
  const [task, setTask] = useState("");

  useEffect(() => {
    const updateTodos = () => setTodos(TodoStore.getTodos());
    TodoStore.on("change", updateTodos);
    return () => TodoStore.removeListener("change", updateTodos);
  }, []);

  const handleAddTodo = () => {
    AppDispatcher.dispatch(addTodo(task));
    setTask("");
  };

  return (
    <div>
      <h2>Todo List</h2>
      <input value={task} onChange={(e) => setTask(e.target.value)} />
      <button onClick={handleAddTodo}>Add Task</button>
      <ul>{todos.map((todo, index) => <li key={index}>{todo}</li>)}</ul>
    </div>
  );
};

export default TodoApp;

7. Mention the advantages of using Suspense in React?

React's Suspense component enhances user experience by improving how components handle asynchronous rendering.

Key Advantages:

• Code Splitting & Lazy Loading – Allows loading components dynamically with React.lazy(), reducing initial load time.
• Efficient Data Fetching – Works with React Server Components and upcoming features like React 18’s Concurrent Mode.
• Fallback UI – Displays a loading indicator while content is being fetched, preventing UI flickering.

8. Design a component that demonstrates the implementation of React portals.

React Portals allow rendering components outside the main DOM hierarchy, which is useful for modals, tooltips, and popups.

Example: Creating a Modal with React Portals

import React from "react";
import ReactDOM from "react-dom";

const Modal = ({ isOpen, onClose, children }) => {
  if (!isOpen) return null;
  
  return ReactDOM.createPortal(
    <div style={{ position: "fixed", top: 0, left: 0, width: "100%", height: "100%", background: "rgba(0,0,0,0.5)", display: "flex", alignItems: "center", justifyContent: "center" }}>
      <div style={{ background: "white", padding: "20px", borderRadius: "5px" }}>
        {children}
        <button onClick={onClose}>Close</button>
      </div>
    </div>,
    document.getElementById("portal-root")
  );
};

export default Modal;

In index.html, add:

<div id="portal-root"></div>

9. Construct a React component that employs error boundaries to handle potential errors in the component tree.

Error Boundaries catch errors in React component trees and prevent the entire app from crashing.

Example: Implementing an Error Boundary Component

import React, { Component } from "react";

class ErrorBoundary extends Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false };
  }

  static getDerivedStateFromError(error) {
    return { hasError: true };
  }

  componentDidCatch(error, errorInfo) {
    console.error("Error caught:", error, errorInfo);
  }

  render() {
    if (this.state.hasError) {
      return <h2>Something went wrong!</h2>;
    }
    return this.props.children;
  }
}

export default ErrorBoundary;

Usage in App:

import ErrorBoundary from "./ErrorBoundary";
import FaultyComponent from "./FaultyComponent";

const App = () => (
  <ErrorBoundary>
    <FaultyComponent />
  </ErrorBoundary>
);

10. How would you integrate a loading spinner in your application to indicate data fetching in progress?

Loading spinners enhance user experience by indicating that data is being fetched.

Example: Adding a Loading Spinner While Fetching Data

import React, { useState, useEffect } from "react";

const LoadingSpinner = () => (
  <div style={{ textAlign: "center", margin: "20px" }}>
    <div className="loader"></div>
    <p>Loading...</p>
  </div>
);

const DataFetchingComponent = () => {
  const [data, setData] = useState(null);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    setTimeout(() => {
      setData(["Item 1", "Item 2", "Item 3"]);
      setLoading(false);
    }, 2000);
  }, []);

  return (
    <div>
      {loading ? <LoadingSpinner /> : <ul>{data.map((item, i) => <li key={i}>{item}</li>)}</ul>}
    </div>
  );
};

export default DataFetchingComponent;

Adding CSS for Spinner:

.loader {
  border: 4px solid #f3f3f3;
  border-top: 4px solid #3498db;
  border-radius: 50%;
  width: 30px;
  height: 30px;
  animation: spin 1s linear infinite;
}

@keyframes spin {
  0% { transform: rotate(0deg); }
  100% { transform: rotate(360deg); }
}

The demand for skilled ReactJS developers in India continues to rise, with numerous companies actively seeking talent in this domain. Here are some top companies you might consider when applying for ReactJS positions:

1. Uplers

Uplers is a prominent ReactJS development company known for delivering high-quality web applications. They offer opportunities to work on diverse projects, enhancing your skills in a dynamic environment.

2. GeekyAnts

Specializing in web and mobile app development, GeekyAnts provides a platform for ReactJS developers to engage in innovative projects. Their focus on cutting-edge technologies makes them a desirable workplace for developers aiming to grow their expertise.

3. Stack Console

Stack Console is among the top companies in India currently hiring ReactJS developers. They offer roles that involve working on scalable and robust web applications, providing a great opportunity to apply and expand your ReactJS skills.

4. MavenAI Technologies

MavenAI Technologies is actively seeking ReactJS developers to join their team. Working here would involve developing intelligent solutions, allowing you to integrate AI with ReactJS, thus broadening your development experience.

5. Heliverse Technologies

Heliverse Technologies offers positions for ReactJS developers interested in creating innovative web solutions. Their commitment to technology and development provides a conducive environment for professional growth.

6. Quytech

Quytech is another company on the lookout for skilled ReactJS developers. They focus on developing custom software solutions, offering a variety of projects that can help enhance your development skills.

7. Flipkoins

Flipkoins is actively hiring ReactJS developers to work on their innovative platforms. Joining their team would provide an opportunity to work in a fast-paced environment, contributing to cutting-edge projects.

8. Aalpha Information Systems

Aalpha Information Systems is a renowned ReactJS development company in India. They offer custom React development services and are known for their expertise in API and web service integration. Working with Aalpha can provide exposure to a variety of projects, enhancing your development skills.

9. RushKar

RushKar is a top-rated ReactJS development company offering opportunities for developers to work on diverse projects. They emphasize cost-effective and timely delivery, making it an excellent place to apply your ReactJS knowledge in real-world scenarios.

10. iCoderz Solutions

iCoderz Solutions is known for hiring top ReactJS specialists to build high-quality web applications. They offer a platform to work on dynamic and interactive user interfaces, providing a significant boost to your development career.

When applying to these companies, ensure your portfolio showcases your proficiency in ReactJS, including any projects or contributions to open-source platforms. Tailoring your resume to highlight relevant experience can significantly enhance your chances of securing a position in these esteemed organizations.

Learning these React JS coding test questions and answers and React coding interview questions will prepare you for interviews at all levels. Understanding these concepts will boost your confidence if you're a beginner learning about components or an expert diving into architecture patterns. Start practising today to excel in your next interview!

1. What is JSX in React? 

JSX stands for JavaScript XML and is a syntax extension that allows you to write HTML-like code within JavaScript. It makes it easier to define UI components and visualize the UI structure in React.

2. What is the difference between state and props in React? 

State manages data within a component and can be updated over time, triggering re-renders. Props are read-only and passed from a parent to a child component, helping share data and behaviour.

3. Explain the concept of "lifting state up" in React. 

Lifting state up refers to moving the state to the nearest common ancestor of components that need access to the data. This approach helps in managing the shared state between sibling components.

4. What are React Hooks? 

React Hooks are functions that allow functional components to manage state and side effects. Some commonly used hooks are useState, useEffect, and useContext, which help in maintaining state and performing side effects in functional components.

5. What is React's Context API? 

The Context API allows you to pass data through the component tree without prop drilling. It is often used to manage the global state across multiple components, like authentication or theme settings.

6. How does React handle events? 

React handles events using camelCase syntax, such as onClick, onChange, etc. Event handlers are passed as functions, and React takes care of normalising events across browsers.

7. What is the Virtual DOM in React? 

The Virtual DOM is an in-memory representation of the actual DOM. React uses it to compare changes between the current and previous states of the UI, making efficient updates to the actual DOM by performing a "diffing" process.