Advanced React JS Interview Questions

On April 25, 2025, Posted by CRS Info Solutions , In Reactjs, With Comments Off

What are Higher-Order Components (HOCs) in React?
How can you implement code splitting in a React application?
What are React Portals, and when would you use them?
What is the purpose of the Strict Mode in React?
How can you manage render props in React components?
How do you handle asynchronous rendering in React?
What is the role of concurrent mode in React?
How does React Hydration work?
How do you handle internationalization (i18n) in a React application?
How would you optimize React for mobile performance?

Are you ready to tackle advanced React JS interview questions and land your dream role? As a seasoned React developer, you know the basics won’t cut it in a competitive interview. Interviewers want to see your expertise in handling complex challenges, such as optimizing performance, implementing scalable state management with Redux or Context API, and leveraging advanced hooks like useMemo and useCallback. They may dive into topics like server-side rendering with Next.js, React’s reconciliation process, and designing reusable components with custom hooks. These are not just buzzwords—they are the skills that set top-tier React developers apart.

In this guide, I’ve compiled the most critical React JS interview questions to help you prepare for your next big opportunity. You’ll find clear explanations, scenario-based challenges, and practical examples designed to enhance your confidence and problem-solving skills. By mastering these topics, you won’t just answer questions—you’ll demonstrate your ability to think critically and deliver exceptional solutions. Let’s dive in and make sure you’re fully prepared to impress and excel in your interview!

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1. What are Higher-Order Components (HOCs) in React?

A Higher-Order Component (HOC) in React is a function that takes a component as an argument and returns a new component. It’s a design pattern used to reuse component logic, making it easier to handle cross-cutting concerns like authentication, logging, or data fetching. HOCs don’t modify the original component; instead, they wrap it, providing additional functionality while keeping the original component intact. This pattern leverages React’s compositional nature to enhance component behavior.

I often use HOCs when I need to apply the same logic across multiple components. For instance, I can create an HOC for handling authentication checks and wrap any components that require restricted access. However, while HOCs are powerful, they can sometimes make debugging harder due to the added layers. That’s why understanding how to structure HOCs properly is essential for maintaining a clean and efficient codebase.

2. How do HOCs differ from Render Props?

While both HOCs and Render Props help in reusing component logic, they approach the problem differently. HOCs wrap a component and provide additional functionality by injecting props, while Render Props rely on a function prop to share logic between components. With Render Props, you control what to render by passing a function to the child component, making it highly flexible.

For example, in a Render Prop pattern, I might use a component that handles mouse tracking and pass a render function to display the cursor position. On the other hand, an HOC could achieve the same result by wrapping the component and injecting the cursor position as props. While Render Props offer more explicit control, HOCs can simplify complex logic. The choice often depends on the use case and readability requirements.

3. How can you implement code splitting in a React application?

Code splitting is an important optimization technique in React to improve application performance by loading only the code needed for a particular page or feature. I achieve this using React’s React.lazy() and Suspense for component-level splitting or dynamic imports for libraries. This ensures that my app doesn’t load unnecessary code upfront, which can significantly reduce the initial load time.

Here’s a simple example:

import React, { lazy, Suspense } from 'react';  
const LazyComponent = lazy(() => import('./MyComponent'));  

function App() {  
  return (  
    <Suspense fallback={<div>Loading...</div>}>  
      <LazyComponent />  
    </Suspense>  
  );  
}

In this example, React.lazy() dynamically loads MyComponent only when it’s needed. Wrapping it with Suspense provides a fallback UI until the component finishes loading. I also use tools like Webpack to configure advanced splitting strategies, such as splitting code by route or vendor libraries, to further optimize load times.

4. What are React Portals, and when would you use them?

React Portals allow me to render a component’s output outside of its parent DOM hierarchy. This is incredibly useful when I need to create elements like modals, tooltips, or dropdowns that must visually break out of their parent container due to styling or positioning constraints.

For instance, if I’m building a modal and rendering it within its parent, CSS overflow: hidden on the parent might prevent the modal from displaying properly. By using a portal, I can render the modal at the root level of the DOM while keeping it logically tied to its parent React component.

Here’s an example of creating a portal:

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

function Modal({ children }) {  
  return ReactDOM.createPortal(  
    <div className="modal">{children}</div>,  
    document.getElementById('modal-root')  
  );  
}

I’d use this Modal component to ensure proper positioning and styling, regardless of its parent. Portals make such scenarios manageable without compromising React’s declarative nature.

5. What is the purpose of the Strict Mode in React?

Strict Mode is a development tool in React that helps me write better, more reliable code. It doesn’t render anything visible in the UI but provides additional checks and warnings for potential issues in my application. By wrapping components in <React.StrictMode>, I can identify unsafe lifecycle methods, detect side effects in render phases, and ensure I’m following best practices.

For example, Strict Mode highlights deprecated methods like componentWillMount or componentWillReceiveProps, which are often sources of bugs. It also ensures that components are resilient to future updates by running certain functions twice in development to catch side effects.

Using Strict Mode is especially helpful during development, as it helps me future-proof my code by adhering to React’s evolving standards. While it doesn’t impact production builds, it significantly improves the quality and maintainability of applications during the development phase.

6. How can you manage render props in React components?

In React, managing render props involves passing a function as a prop to a component, which allows dynamic rendering of the child content. I often use render props when I need to share reusable logic between components while maintaining control over how the UI is rendered. This pattern is especially useful in cases like tracking state, handling mouse events, or implementing animations.

For example, I can create a MouseTracker component that uses render props to provide the current mouse position:

function MouseTracker({ render }) {  
  const [position, setPosition] = React.useState({ x: 0, y: 0 });  
  const handleMouseMove = (e) => setPosition({ x: e.clientX, y: e.clientY });  

  return <div onMouseMove={handleMouseMove}>{render(position)}</div>;  
}

I can then use it like this:

<MouseTracker render={({ x, y }) => <h1>Mouse is at ({x}, {y})</h1>} />

By managing render props, I ensure reusability and flexibility in how components consume shared logic.

7. How do you implement error boundaries using a class component?

Implementing error boundaries in React requires using a class component that defines the componentDidCatch and getDerivedStateFromError lifecycle methods. These methods allow me to catch and handle JavaScript errors that occur in the rendering process or lifecycle methods of child components. Error boundaries are particularly useful for preventing an entire app from crashing due to an error in a single part of the UI.

Here’s an example of an error boundary:

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

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

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

  render() {  
    if (this.state.hasError) {  
      return <h1>Something went wrong.</h1>;  
    }  
    return this.props.children;  
  }  
}

To use this boundary, I wrap any component that might throw an error with <ErrorBoundary>. This ensures that errors are gracefully handled without affecting the rest of the application.

8. What is the concept of compound components in React?

The compound component pattern in React allows me to create components that work together as a group. It’s a great way to build flexible and reusable UI components by splitting them into smaller parts that share a common state or behavior. With this pattern, I can provide a parent component to manage the state and pass down the relevant data or callbacks to its child components.

For example, I might create a Toggle component with compound children like Toggle.Button and Toggle.On:

const ToggleContext = React.createContext();  

function Toggle({ children }) {  
  const [on, setOn] = React.useState(false);  
  const toggle = () => setOn((prev) => !prev);  

  return (  
    <ToggleContext.Provider value={{ on, toggle }}>  
      {children}  
    </ToggleContext.Provider>  
  );  
}  

Toggle.Button = function Button() {  
  const { toggle } = React.useContext(ToggleContext);  
  return <button onClick={toggle}>Toggle</button>;  
};  

Toggle.On = function On({ children }) {  
  const { on } = React.useContext(ToggleContext);  
  return on ? children : null;  
};

This approach makes it easy for me to design highly composable components where the parent coordinates the behavior, and the children focus solely on presentation.

9. How do you build reusable components in React?

Building reusable components in React involves designing components that are flexible, composable, and decoupled from specific use cases. I ensure this by focusing on three key principles: props for customization, clear separation of concerns, and avoiding tightly coupled dependencies. For example, instead of hardcoding styles or logic, I expose them via props or context.

Let’s say I want to create a reusable Button component. I ensure it can handle different variations (e.g., primary, secondary) by using props:

function Button({ variant = "primary", children, ...props }) {  
  const className = `btn ${variant}`;  
  return <button className={className} {...props}>{children}</button>;  
}

Additionally, I ensure my reusable components are well-documented with clear prop definitions, making them easy to integrate into different parts of the application. Writing tests for these components also ensures their reliability across use cases.

10. How do you handle asynchronous rendering in React?

Handling asynchronous rendering in React requires managing promises and ensuring the UI remains consistent while waiting for updates. I commonly use tools like useEffect and Suspense to manage side effects and async data fetching effectively. With asynchronous rendering, React allows me to prioritize rendering tasks and provide smooth user experiences.

One way I handle this is by using useEffect for fetching data and maintaining a loading state:

function DataFetcher({ url }) {  
  const [data, setData] = React.useState(null);  
  const [loading, setLoading] = React.useState(true);  

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

  if (loading) return <p>Loading...</p>;  
  return <pre>{JSON.stringify(data, null, 2)}</pre>;  
}

For advanced cases like concurrent rendering, I leverage React’s Suspense and React.lazy to handle component-level async operations gracefully. This ensures that the app remains responsive even when waiting for data or components to load.

11. What is React Fiber, and how has it changed React’s architecture?

React Fiber is a reimplementation of React’s core algorithm introduced in React 16. It’s designed to improve the way React handles rendering and updating the UI by breaking rendering work into small units, known as “fibers.” This allows React to pause and resume rendering tasks, giving it the ability to prioritize updates like user interactions over less critical tasks, such as background data fetching. I appreciate how Fiber enables smoother and more responsive user interfaces, especially in apps with complex UIs or animations.

Before Fiber, React used a stack-based algorithm that processed updates in a single pass, which could block the UI if the work took too long. With Fiber, React can split rendering into incremental chunks and distribute the workload over multiple frames, leveraging asynchronous rendering. This architectural change introduced concurrent mode, which optimizes React’s ability to handle heavy workloads without sacrificing performance.

12. How do you implement SSR (Server-Side Rendering) with React?

Server-Side Rendering (SSR) in React involves rendering React components on the server and sending the resulting HTML to the client. This improves performance by reducing the time to the first paint and enhances SEO by delivering pre-rendered content to search engines. To implement SSR, I typically use frameworks like Next.js or the built-in renderToString method from React’s react-dom/server package.

For example, I can create an SSR setup with renderToString:

import React from "react";  
import ReactDOMServer from "react-dom/server";  
import App from "./App";  

const express = require("express");  
const app = express();  

app.get("/", (req, res) => {  
  const html = ReactDOMServer.renderToString(<App />);  
  res.send(`  
    <!DOCTYPE html>  
    <html>  
      <head><title>SSR Example</title></head>  
      <body><div id="root">${html}</div></body>  
    </html>  
  `);  
});  

app.listen(3000, () => console.log("Server running on http://localhost:3000"));

The ReactDOMServer.renderToString function generates HTML on the server, enabling faster initial page loads and better SEO as the browser renders preloaded HTML.

13. How can you create a custom hook in React?

Creating a custom hook in React allows me to extract reusable logic from components and share it across multiple components. Custom hooks start with the use prefix, just like React’s built-in hooks, and leverage the same lifecycle and state management functionalities provided by hooks like useState and useEffect.

For instance, I might create a useFetch hook for fetching data:

import { useState, useEffect } from "react";  

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

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

  return { data, loading };  
}

By using useFetch, I can simplify components and reduce redundancy when handling similar tasks. Custom hooks help in maintaining clean and modular code.

14. What is the role of concurrent mode in React?

Concurrent mode in React enhances the user experience by enabling the application to handle multiple rendering tasks simultaneously without blocking the main thread. I find it especially useful for creating apps that remain responsive during heavy updates or data fetching. With concurrent mode, React can pause, prioritize, and resume rendering tasks as needed, ensuring smoother transitions and better interactivity.

For instance, when a user types in a search box, React can prioritize rendering the updated input value over other background tasks, such as loading search results. The Suspense API is one feature of concurrent mode that helps manage loading states efficiently by deferring rendering until a promise resolves. Here’s an example:

const SearchResults = React.lazy(() => import("./SearchResults"));  

function App() {  
  return (  
    <React.Suspense fallback={<div>Loading...</div>}>  
      <SearchResults />  
    </React.Suspense>  
  );  
}

With concurrent mode, I can build highly interactive UIs without worrying about performance bottlenecks caused by heavy rendering tasks.

15. How does React Hydration work?

Hydration in React refers to the process of attaching React’s event listeners to the server-rendered HTML on the client side. This step is essential for turning a pre-rendered static HTML page into a fully functional React application, enabling client-side interactivity. I’ve used hydration in projects where Server-Side Rendering (SSR) is implemented to improve initial page load performance and SEO.

When a React app is server-rendered, the server sends HTML to the client. React then uses the ReactDOM.hydrate method to “hydrate” this HTML, adding event listeners and state management without re-rendering the entire page. For example:

import React from "react";  
import ReactDOM from "react-dom";  
import App from "./App";  

ReactDOM.hydrate(<App />, document.getElementById("root"));

Hydration ensures that the rendered output on the client matches the server-rendered HTML. If there are discrepancies, React may re-render the mismatched parts to ensure consistency. This process helps maintain the performance benefits of SSR while enabling a seamless transition to a fully interactive client-side React app.

See more: TCS AngularJS Developer Interview Questions

16. How do you handle reactive data fetching in React?

Handling reactive data fetching in React involves dynamically fetching and updating data based on user interactions or changes in application state. I typically use hooks like useEffect for managing side effects and useState to store fetched data. For example, when a user selects a filter, I trigger a re-fetch of data matching the new criteria.

To make the process more seamless, I often incorporate libraries like React Query or Axios. React Query, for instance, simplifies state management and caching for fetched data. It automatically re-fetches stale data and handles retries, making my applications more resilient. With React Query, fetching reactive data might look like this:

import { useQuery } from "react-query";  

function fetchData(key, query) {  
  return fetch(`https://api.example.com/data?search=${query}`).then(res => res.json());  
}  

function App({ searchQuery }) {  
  const { data, isLoading } = useQuery(["data", searchQuery], () => fetchData(searchQuery));  

  if (isLoading) return <div>Loading...</div>;  
  return <div>{data.map(item => <p key={item.id}>{item.name}</p>)}</div>;  
}

The code snippet demonstrates reactive data fetching using React Query, which simplifies API calls and handles caching, retries, and state management. The useQuery hook fetches data whenever the searchQuery changes. The isLoading state ensures a loading indicator is displayed during the fetch. The approach keeps the data-fetching logic clean and reactive to user input.

17. What is React Suspense, and how do you use it with lazy loading?

React Suspense is a feature that helps manage the loading state of components that depend on asynchronous operations, such as dynamic imports or data fetching. I use it alongside React.lazy for lazy loading components, improving initial load times by splitting the application into smaller, on-demand chunks.

For example, if I want to load a heavy component only when needed, I use React.lazy with Suspense:

import React, { Suspense } from "react";  

const HeavyComponent = React.lazy(() => import("./HeavyComponent"));  

function App() {  
  return (  
    <Suspense fallback={<div>Loading...</div>}>  
      <HeavyComponent />  
    </Suspense>  
  );  
}

The fallback prop specifies what to display while the component is loading. This setup is especially useful for improving user experience in applications with large or rarely used components.

18. How can you build a progressive web app (PWA) with React?

Building a progressive web app (PWA) with React involves creating a fast, reliable, and installable application. I rely on tools like Create React App (CRA), which has built-in support for PWAs. The core features of a PWA include offline support, a service worker, and a web app manifest for installation.

When I create a React app, I ensure the serviceWorker is registered to enable offline capabilities. This file caches static assets, allowing the app to function without an internet connection. Additionally, I add a manifest.json file to define the app’s name, icons, and splash screen settings.

Here’s an example of service worker registration:

import React from "react";  
import ReactDOM from "react-dom";  
import App from "./App";  
import * as serviceWorker from "./serviceWorker";  

ReactDOM.render(<App />, document.getElementById("root"));  

serviceWorker.register();

This simple setup ensures my React app meets the criteria for a PWA, making it user-friendly and installable on various devices.

19. How do you handle internationalization (i18n) in a React application?

Handling internationalization (i18n) in React means adapting the app to support multiple languages and locales. I often use libraries like react-i18next, which simplifies translation management. With this, I can create a centralized file to store translation strings and dynamically switch languages based on user preferences.

Here’s an example setup:

Install react-i18next and i18next.
Configure translations in a file like en.json or fr.json.
Use the useTranslation hook to fetch and display the correct translations.

import { useTranslation } from "react-i18next";  

function Welcome() {  
  const { t } = useTranslation();  
  return <h1>{t("welcome_message")}</h1>;  
}

This method makes React apps more inclusive and adaptable to global users.

20. How would you optimize React for mobile performance?

Optimizing React apps for mobile performance involves ensuring smooth, fast, and responsive user experiences. I start by reducing bundle sizes using techniques like lazy loading, tree shaking, and minimizing dependencies. I also prioritize compressing assets and using efficient image formats like WebP for faster loading.

Another key focus is using tools like React Native for mobile-specific development or optimizing touch gestures and animations for smoother interactions. I avoid heavy computations in the main thread by delegating them to web workers and use memoization with React.memo and useCallback to prevent unnecessary re-renders.

For instance, to reduce over-rendering, I might write:

const MemoizedComponent = React.memo(({ data }) => {  
  console.log("Rendering...");  
  return <div>{data}</div>;  
});

The code snippet demonstrates the use of React.memo to optimize component re-renders, ensuring the wrapped component renders only when props change. This approach reduces unnecessary computations, especially for frequently updated data. Combined with lazy loading and smaller bundle sizes, it boosts the app’s mobile performance by improving responsiveness and speed.

Conclusion

Excelling in Advanced React JS Interview Questions is a game-changer for developers aiming to stand out in competitive technical interviews. These questions are designed to evaluate your proficiency in handling complex React concepts, optimizing performance, and implementing innovative solutions. Mastering topics like React Fiber, custom hooks, and server-side rendering not only showcases your technical depth but also highlights your ability to build scalable, high-performance applications.

This guide empowers you with the knowledge and practical examples needed to tackle challenging scenarios with confidence. From crafting progressive web apps to handling reactive data fetching, you’ll gain insights that translate directly to real-world development. By preparing with these advanced concepts, you’re not just ready for the interview—you’re ready to impress and secure a top-tier React JS role.

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