React JS Interview Questions for 5 years Experience
React JS Interview Questions for 5 years Experience

React JS Interview Questions for 5 years Experience

On November 5, 2024, Posted by , In Reactjs, With Comments Off on React JS Interview Questions for 5 years Experience
React JS Interview Questions for 5 years Experience

Table Of Contents

React JS is one of the most popular JavaScript libraries. It is used for building dynamic and interactive user interfaces. This is especially true for single-page applications. As a front-end developer with 5 years of experience in React JS, you need to understand this library well. You should know its core principles and advanced features.

You should be skilled in React JS’s component-based architecture. This means you can build applications using small, reusable parts. You should also know how to manage state and props. Using hooks and lifecycle methods is important too. These tools help you write better code and improve performance. If you are preparing for a React JS interview, it is crucial to know these concepts deeply. You should be able to explain them clearly. This shows you can apply what you know in real-life situations.

With five years of experience, interviewers will expect more than just basic knowledge. You might face questions about handling large applications. You may need to discuss using Redux or Context API for state management. Employers look for developers who can solve common React issues and debug efficiently. They also want you to follow industry standards. This includes writing clean and maintainable code. If you can talk about your real-world experience with these advanced concepts, you will have an advantage in your React JS interview.

CRS Info Solutions stands out for its exceptional React JS training in Hyderabad, tailored specifically for students. Their program focuses on practical, hands-on learning, ensuring that students not only understand React js training concepts but also apply them effectively in real-world scenarios. This approach has established CRS Info Solutions as a go-to destination for aspiring React JS developers in the region.

1. What is the virtual DOM, and how does it enhance performance in React?

The virtual DOM is a lightweight copy of the actual DOM. React uses this virtual DOM to manage changes in the user interface efficiently. Whenever the state of a React component changes, React first updates the virtual DOM instead of directly changing the actual DOM. Once the virtual DOM is updated, React compares it with the previous version to see what changes need to be made. This process is called “diffing”. After this comparison, React updates only the necessary parts of the real DOM, instead of re-rendering the entire page.

This approach enhances performance because updating the DOM directly can be slow, especially in complex web applications. By limiting the number of direct DOM manipulations, React ensures that the interface remains responsive and smooth. In large applications with frequent updates, this optimization becomes even more significant, allowing for better user experiences without compromising speed.

2. How does React’s reconciliation process work, and why is it important?

The reconciliation process in React is how the library updates the actual DOM after a change occurs in the virtual DOM. When React detects a change in the state or props of a component, it doesn’t immediately update the real DOM. Instead, it updates the virtual DOM first and then uses the reconciliation algorithm to find out what specific parts of the DOM have changed. The goal is to minimize the number of updates to the real DOM by applying only the necessary changes.

This process is important because it keeps the application running efficiently. React can determine which components need to be re-rendered and which don’t. It also helps maintain smooth transitions and animations by avoiding unnecessary re-renders. For instance, if I update only one small section of the page, React will ensure that only that section is re-rendered, not the entire UI, making the application faster and more scalable.

Explore: React JSX

3. How does React Fiber improve React’s rendering process?

React Fiber is the reimplementation of React’s core algorithm, introduced to improve rendering performance. Unlike the previous version of React, Fiber enables React to break down rendering work into small units, allowing it to pause and resume tasks. This means React can prioritize tasks based on urgency. For example, UI updates that are needed immediately (like user inputs) can be prioritized over less urgent tasks like data fetching.

One of the key improvements in Fiber is its support for asynchronous rendering. In earlier versions of React, rendering was done synchronously, meaning large components or complex UI updates could block the main thread and make the app feel sluggish. Fiber fixes this by allowing React to interrupt and resume work as needed. This results in a smoother user experience, especially in apps with complex animations or heavy data.

Here’s a small code snippet to demonstrate asynchronous rendering in Fiber:

function MyComponent() {
  const [count, setCount] = useState(0);

  // Asynchronous update
  useEffect(() => {
    setTimeout(() => setCount(count + 1), 1000);
  }, [count]);

  return <div>Count: {count}</div>;
}

In this example, the setTimeout function allows the state update to occur after a delay, showing how React can handle state changes in the background without blocking the UI.

Explore: How Can You Pass Props to Children Components in React?

4. Explain the concept of React Hooks and how they differ from class-based components.

This question is very important for 5 years experienced React JS developer. React Hooks are functions that let me use state and other React features in functional components. Before Hooks, we had to use class components to manage state or lifecycle methods like componentDidMount or componentDidUpdate. Hooks eliminate the need for class components by allowing functional components to do everything a class component can do, but in a cleaner and more concise way. The most commonly used hooks are useState and useEffect.

In class components, state management and lifecycle methods can get complicated, especially when dealing with complex logic. Hooks, on the other hand, allow us to break down complex logic into smaller, reusable pieces. For example, I can have multiple useEffect hooks to handle different side effects, while in class components, I would need to handle all side effects in one lifecycle method. This makes code with Hooks easier to read, maintain, and reuse.

Here’s a comparison using class-based components vs Hooks:

Class component:

class MyComponent extends React.Component {
  state = { count: 0 };

  componentDidMount() {
    document.title = `Count: ${this.state.count}`;
  }

  componentDidUpdate() {
    document.title = `Count: ${this.state.count}`;
  }

  render() {
    return (
      <button onClick={() => this.setState({ count: this.state.count + 1 })}>
        Click me
      </button>
    );
  }
}

Hooks version:

function MyComponent() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    document.title = `Count: ${count}`;
  }, [count]);

  return <button onClick={() => setCount(count + 1)}>Click me</button>;
}

In the hooks version, you can see how much simpler the code becomes. The useState and useEffect hooks allow us to manage state and side effects in a much more readable way.

5. What are the differences between useEffect and componentDidMount?

Let’s start looking into this 5 years experienced React JS developer interview question. The **useEffect** hook is React’s way of managing side effects in functional components, whereas componentDidMount is used in class components to execute code after the component mounts. Both serve similar purposes in ensuring that some side effect happens after the component has rendered for the first time, but they work differently under the hood.

With componentDidMount, we can run code only once after the initial render. On the other hand, useEffect can be run multiple times depending on the dependencies you pass to it. If I want useEffect to behave like componentDidMount, I would pass an empty array as the second argument. This ensures the effect runs only once when the component mounts. Here’s an example using both:

Class component with componentDidMount:

class MyComponent extends React.Component {
  componentDidMount() {
    console.log("Component mounted");
  }

  render() {
    return <div>Hello, world!</div>;
  }
}

Functional component with useEffect:

function MyComponent() {
  useEffect(() => {
    console.log("Component mounted");
  }, []);  // empty array makes it run only on mount

  return <div>Hello, world!</div>;
}

The key difference is flexibility. useEffect can be used for more than just mounting— it can also handle updates and cleanups, making it a powerful tool in functional components.

Explore: Explore: Form Handling in React JS

6. Explain React.memo and when to use it.

React.memo is a higher-order component (HOC) that helps optimize functional components by preventing unnecessary re-renders. When we wrap a component with React.memo, it checks whether the props of that component have changed since the last render. If the props haven’t changed, React will skip rendering that component again and use the previously rendered output, saving performance.

I use React.memo in scenarios where a component re-renders frequently but its props don’t change. For example, in a large list where only one item changes at a time, I can wrap the list items with React.memo to ensure that only the changed item re-renders, not the entire list. However, I should be cautious with React.memo—it works best with components that are pure, meaning they rely solely on their props for rendering.

Example:

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

// This will not re-render MyComponent unless the `name` prop changes.

In this example, MyComponent will only re-render if the name prop changes, reducing unnecessary re-renders and improving performance in larger applications. It’s important to remember that React.memo does a shallow comparison of props, so for more complex data structures, I may need to implement custom comparison logic.

7. How does Context API work, and what are its advantages and disadvantages compared to Redux?

The Context API allows me to share state globally across a React application without passing props down through every level of the component tree. It provides a way to avoid prop drilling, where I have to pass props through multiple components that don’t need them. I use React.createContext to create a context, and Context.Provider to pass down the values I want to share. Any component in the tree can access these values by using useContext.

The Context API is simpler to set up compared to Redux, which is a more comprehensive state management solution. While Context works well for smaller applications or simple data like themes or user authentication, it doesn’t provide as much structure as Redux. Redux, with its actions, reducers, and middleware, is designed for managing more complex state logic in large applications. It also provides tools for debugging and performance optimization, which the Context API lacks.

Here’s a simple example of how the Context API works:

const UserContext = React.createContext();

function App() {
  return (
    <UserContext.Provider value="John Doe">
      <UserProfile />
    </UserContext.Provider>
  );
}

function UserProfile() {
  const userName = useContext(UserContext);
  return <div>{userName}</div>;
}

In this example, I’m passing the userName globally to the UserProfile component without having to pass it down as a prop. While Context is useful in cases like this, I would use Redux for managing complex states, such as handling large datasets or when I need advanced features like time-travel debugging.

Explore: Form Handling in React JS

8. Can you describe how to manage side effects in a React application using useEffect?

The useEffect hook is the standard way to handle side effects in functional components. Side effects are operations that happen outside the React rendering flow, such as API calls, subscriptions, or updating the DOM. I use useEffect to perform these side effects at different stages of a component’s lifecycle.

To manage side effects properly, I pass two arguments to useEffect. The first argument is a function that contains the side effect logic, such as fetching data from an API. The second argument is an optional dependencies array. This array tells React when to re-run the effect. If I leave the array empty, the effect runs only once, similar to componentDidMount in class components. If I include variables in the array, React will re-run the effect whenever those variables change.

For example, fetching data using useEffect:

function DataFetcher() {
  const [data, setData] = useState(null);

  useEffect(() => {
    fetch("https://api.example.com/data")
      .then(response => response.json())
      .then(data => setData(data));
  }, []); // Empty array ensures the effect runs only once

  return <div>{data ? JSON.stringify(data) : "Loading..."}</div>;
}

In this case, the useEffect hook fetches data only once when the component mounts, and updates the component state with the fetched data. I can also use the cleanup function within useEffect to handle things like canceling subscriptions or cleaning up resources when the component unmounts, which is essential to prevent memory leaks in long-running applications.

Explore: React Router Interview Questions

9. What are Higher-Order Components (HOC), and how do they differ from Render Props?

A Higher-Order Component (HOC) is a pattern in React where one component wraps another to enhance or extend its functionality. I create an HOC by defining a function that takes a component as an argument and returns a new component with additional props or logic. HOCs are typically used for reusing component logic like authentication checks, data fetching, or injecting props.

For example, an HOC might look like this:

function withAuth(Component) {
  return function AuthComponent(props) {
    const isAuthenticated = true; // Simplified logic
    return isAuthenticated ? <Component {...props} /> : <div>Please log in</div>;
  };
}

const Dashboard = withAuth(function(props) {
});

In this example, the withAuth HOC wraps the Dashboard component, checking if the user is authenticated before rendering the content. HOCs are useful when I need to reuse the same logic across multiple components, as I can apply them without modifying the component itself.

Render Props, on the other hand, is a pattern where I pass a function as a prop to a component and then use that function to dynamically render UI. It allows me to share code between components by having the render logic be flexible and customizable through a function prop.

For example:

function DataFetcher({ render }) {
  const [data, setData] = useState(null);

  useEffect(() => {
    fetch("https://api.example.com/data")
      .then(response => response.json())
      .then(data => setData(data));
  }, []);

  return render(data);
}

function App() {
  return (
    <DataFetcher render={data => <div>{data ? JSON.stringify(data) : "Loading..."}</div>} />
  );
}

In this example, the DataFetcher component receives a render prop and dynamically renders the UI based on the fetched data. The key difference between HOCs and Render Props is that HOCs wrap components to extend their behavior, while Render Props allow the parent component to control rendering through a function.

Explore: React Hooks: Revolutionizing Functional Components

10. How would you implement lazy loading in a React application?

Lazy loading in React is a technique I use to load components only when they are needed. It’s an essential optimization for improving the performance of large applications by reducing the initial bundle size. React makes lazy loading easy with the React.lazy function, which allows components to be dynamically imported only when they are rendered.

Here’s a basic example of how I can implement lazy loading:

const LazyComponent = React.lazy(() => import('./MyComponent'));

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

In this example, React.lazy is used to load MyComponent only when it’s rendered. The Suspense component acts as a placeholder while the lazy-loaded component is being fetched, displaying a fallback UI like a loading spinner. This approach ensures that non-critical components aren’t loaded until they’re needed, improving the user experience by speeding up the initial load time.

I use lazy loading when dealing with code splitting, where I split the application into smaller bundles that are loaded on demand. This technique is crucial when building large applications with lots of routes or complex UI, ensuring faster load times and better performance.

11. Explain how to use React Router to manage navigation in a single-page application.

React Router is a library that helps me manage navigation in a single-page application (SPA) by allowing me to define routes and link components to specific URLs. In SPAs, the browser doesn’t reload the page when navigating between sections. Instead, React Router dynamically loads different components based on the URL, making the app feel more fluid and responsive.

To use React Router, I import components like BrowserRouter, Route, and Link. BrowserRouter is the parent component that wraps around the entire application, enabling route management. The Route component defines the path for each route, and Link allows me to create navigation links without causing a page refresh.

Here’s an example:

eimport { BrowserRouter as Router, Route, Link } from 'react-router-dom';

function App() {
  return (
    <Router>
      <nav>
        <Link to="/home">Home</Link>
        <Link to="/about">About</Link>
      </nav>
      <Route path="/home" component={Home} />
      <Route path="/about" component={About} />
    </Router>
  );
}

In this example, I’m defining two routes: one for /home and one for /about. When the user clicks on the links, React Router changes the URL and loads the corresponding component, all without reloading the page. I also use Switch to ensure that only one route is rendered at a time, making the routing more efficient and preventing multiple components from rendering.

Explore: Lifecycle Methods in React

12. How do you handle error boundaries in React, and why are they important?

Error boundaries in React are components that catch JavaScript errors in their child component tree and display a fallback UI instead of crashing the entire application. I use them to handle errors gracefully, ensuring the user experience isn’t completely disrupted by an error in one part of the app.

To create an error boundary, I need to use a class component because error boundaries rely on the lifecycle method componentDidCatch. When an error is caught, I can display a custom error message or fallback UI while logging the error for debugging purposes.

Here’s an example of a basic 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 in ErrorBoundary:", error, info);
  }

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

In this example, the ErrorBoundary component catches errors in its child components and displays a fallback UI. Error boundaries are essential for production environments, where I want to prevent the entire app from breaking due to an uncaught error. However, they don’t catch errors that occur in event handlers or asynchronous code, so I may need to use try...catch blocks for those scenarios.

13. What are controlled and uncontrolled components in React? When would you use each?

In React, controlled components are form elements like input fields where the value is controlled by React’s state. I keep the form’s state in the component’s state object and update it through event handlers. The form element’s value is always in sync with the state, allowing React to fully manage the input.

Here’s an example of a controlled component:

function ControlledForm() {
  const [inputValue, setInputValue] = useState('');

  return (
    <input 
      value={inputValue} 
      onChange={e => setInputValue(e.target.value)} 
    />
  );
}

In this example, the input’s value is controlled by the inputValue state. As the user types, the onChange handler updates the state, keeping the input’s value in sync with React. Controlled components give me full control over form behavior, making validation and submission logic easier to implement.

Uncontrolled components, on the other hand, store their own state internally. Instead of using React state to manage the form’s values, I use refs to directly access the DOM elements and retrieve their values. Uncontrolled components are simpler when I don’t need to manage or validate the form’s data in real-time.

Here’s an example of an uncontrolled component:

function UncontrolledForm() {
  const inputRef = useRef(null);

  const handleSubmit = () => {
    console.log(inputRef.current.value); // Accessing the input value via ref
  };

  return <input ref={inputRef} />;
}

In this case, I’m using useRef to access the value of the input field when needed. Uncontrolled components are useful when I want a quick and simple form, but they are less flexible when it comes to validation or custom behavior.

Explore: Component Composition in React

14. Describe the purpose and use cases of the useReducer hook.

The useReducer hook is an alternative to useState for managing more complex state logic. Instead of directly updating the state, useReducer uses a reducer function to manage state transitions. This makes it particularly useful when I need to handle multiple state changes based on specific actions. This is valuable question for a 5 year React developer.

The reducer function takes the current state and an action, then returns a new state based on the action type. This approach is similar to how state management works in Redux, where actions describe how the state should change, and reducers specify how the changes are applied.

Here’s an example:

const initialState = { count: 0 };

function reducer(state, action) {
  switch (action.type) {
    case 'increment':
      return { count: state.count + 1 };
    case 'decrement':
      return { count: state.count - 1 };
    default:
      return state;
  }
}

function Counter() {
  const [state, dispatch] = useReducer(reducer, initialState);

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

In this example, the reducer function handles two actions, increment and decrement, that modify the state. I use dispatch to send actions, and the state updates accordingly. I’d use useReducer in situations where state logic is too complex for useState, such as managing multiple form inputs or implementing complex workflows.

Explore: Conditional Rendering in React

15. How does React Fiber improve React’s rendering process?

React Fiber is a re-implementation of React’s rendering engine designed to make React more responsive to asynchronous updates. Fiber breaks down the rendering process into small, manageable units of work, allowing React to prioritize updates based on their importance.

Before Fiber, React’s rendering process was synchronous, meaning large updates could block the main thread and slow down the app. With Fiber, React can interrupt and pause less important tasks, such as background data fetching or low-priority UI updates, to focus on more urgent tasks like user interactions.

One of the main benefits of Fiber is its ability to prioritize rendering. For example, if I have a list of thousands of items, React can split the rendering work over multiple frames instead of freezing the UI to render everything at once. This results in a smoother user experience, especially for applications with heavy workloads like animations or data visualizations.

Another improvement in Fiber is concurrent rendering, which enables React to render multiple components simultaneously, further optimizing the application’s performance. With Fiber, React can now pause and resume rendering as needed, ensuring that the app stays responsive under heavy load.

16. What is the Strict Mode in React, and how does it help in identifying potential issues?

Strict Mode in React is a tool I use to identify potential problems in my application during the development phase. It doesn’t affect the app’s behavior in production but helps catch common mistakes by performing additional checks on components. Strict Mode makes the app more robust and future-proof by highlighting issues that might arise with upcoming React features.

When I wrap my component tree in <React.StrictMode>, React enables additional checks, such as verifying deprecated lifecycle methods and ensuring that side effects are handled correctly. For example, it ensures I’m not using unsafe practices like mutating state directly or performing side effects in the wrong lifecycle methods.

Here’s how I use it:

<React.StrictMode>
  <App />
</React.StrictMode>

React also double-invokes some functions like useEffect in Strict Mode to help detect any issues with side effects. While this might seem annoying during development, it’s a valuable way to catch subtle bugs that could otherwise go unnoticed. Overall, Strict Mode is an essential tool to ensure best practices and prepare my app for future versions of React.

Explore: Step-by-Step Guide to React’s Context API

17. How can you manage forms efficiently in React? What are some common libraries for form handling?

In React, managing forms efficiently requires me to keep track of the form’s state, handle input changes, and manage form submissions. I often use controlled components where I control the input values through React state. Each form field is tied to a state variable, and every change updates the state accordingly.

For more complex forms with many fields or validation requirements, I turn to libraries that streamline the process. The most popular libraries for managing forms in React are:

  1. Formik: It provides simple ways to handle forms, manage validation, and keep track of the form’s state. It works well with controlled components and supports both synchronous and asynchronous validation.
  2. React Hook Form: It’s built around uncontrolled components and uses refs to access input values directly. It’s highly performant and lightweight, making it an excellent choice for larger forms where performance is critical.
  3. Yup: This is often used in conjunction with Formik to handle schema-based validation. It helps define validation rules for each form field, making it easier to enforce strict validation logic.

Here’s a small example using React Hook Form:

import { useForm } from 'react-hook-form';

function MyForm() {
  const { register, handleSubmit } = useForm();

  const onSubmit = data => {
    console.log(data);
  };

  return (
    <form onSubmit={handleSubmit(onSubmit)}>
      <input {...register("username")} />
      <input {...register("email")} />
      <button type="submit">Submit</button>
    </form>
  );
}

In this example, useForm helps manage form state with minimal effort. It’s highly performant because it uses refs instead of managing inputs through state. Using a library like React Hook Form can reduce boilerplate code, especially in larger applications where managing form state manually becomes cumbersome.

Read more : Conditional Rendering in React JS

18. What are React portals, and when would you use them?

React portals provide a way for me to render a child component into a DOM node that exists outside the normal parent-child hierarchy of the React component tree. This is useful when I want to render UI elements, such as modals, tooltips, or dropdowns, outside the main React root component to avoid issues like CSS z-index conflicts.

To create a portal, I use ReactDOM.createPortal, which allows me to specify a target DOM node. Even though the portal renders outside the parent component in the DOM, React still keeps the component’s state and props connected to the parent.

Here’s an example of a modal implemented using a portal:

import ReactDOM from 'react-dom';

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

In this example, the Modal component renders its children into a modal-root DOM node outside of the main React app’s DOM tree. Portals are especially useful when I need to handle modals, overlays, or any UI component that needs to break free from the usual parent-child structure in the DOM. They allow me to ensure that my components are rendered properly without messing up the CSS or other layout constraints.

19. Explain the concept of prop drilling and how to avoid it.

This question is very important for 5 years experienced React JS developer. Prop drilling refers to the situation where I have to pass props down through several layers of components just to get data to a deeply nested component. This can make my code more difficult to manage, as I need to manually pass props through components that don’t even need them, just to get them to the component that does.

For example, if I have a deeply nested component that needs user data, I might have to pass the user prop through multiple components, which makes the code harder to read and maintain:

function App() {
  const user = { name: 'John' };
  return <Parent user={user} />;
}

function Parent({ user }) {
  return <Child user={user} />;
}

function Child({ user }) {
  return <GrandChild user={user} />;
}

function GrandChild({ user }) {
  return <div>Hello, {user.name}!</div>;
}

To avoid prop drilling, I can use Context API or state management libraries like Redux. The Context API allows me to share state across multiple components without manually passing props through each level. I create a context and wrap my components with a Provider, allowing the deeply nested component to consume the context without the need for prop drilling.

Here’s how I would do it with the Context API:

const UserContext = React.createContext();

function App() {
  const user = { name: 'John' };
  return (
    <UserContext.Provider value={user}>
      <Parent />
    </UserContext.Provider>
  );
}

function GrandChild() {
  const user = useContext(UserContext);
  return <div>Hello, {user.name}!</div>;
}

Now, I avoid passing the user prop down through multiple components, simplifying the code and making it easier to maintain. This approach is especially useful in larger applications where data needs to be accessed by many components at different levels of the tree.

Explore: Understanding React.js Props and State with Practical Examples

20. How do you test React components, and what tools/libraries do you prefer for testing?

To test React components, I use unit tests to verify that individual components function as expected and integration tests to ensure that components work together correctly. The most commonly used testing tools in React include Jest, React Testing Library, and Enzyme.

  1. Jest: This is a JavaScript testing framework that allows me to write tests easily and efficiently. It comes with features like mocking, snapshot testing, and parallel test execution. I use Jest to test the output of my components and check if they render the correct UI.
  2. React Testing Library (RTL): RTL focuses on testing the component’s behavior from a user’s perspective. Instead of testing implementation details like method calls, it tests how the UI behaves when the user interacts with it. I prefer RTL because it encourages writing more maintainable and realistic tests.
  3. Enzyme: This is a utility that allows me to shallow render components, making it easy to test individual components in isolation without worrying about their children. However, with the rise of RTL, I tend to use Enzyme less frequently.

Here’s an example of a simple test using React Testing Library:

import { render, screen, fireEvent } from '@testing-library/react';
import '@testing-library/jest-dom';
import Counter from './Counter';

test('increments counter when button is clicked', () => {
  render(<Counter />);
  
  const button = screen.getByText('Increment');
  fireEvent.click(button);
  
  expect(screen.getByText('Count: 1')).toBeInTheDocument();
});

In this example, I’m testing a Counter component. I simulate a button click using fireEvent, and then check if the UI updates correctly by asserting that the count is displayed as 1. By using Jest and React Testing Library together, I can ensure that my components behave as expected and that they work well in different scenarios.

21. Can you explain the difference between React Fragment and a div wrapper?

React Fragment and div wrappers both allow me to group multiple elements inside a single component without breaking the JSX rules that require one parent element. However, they serve different purposes and have some important differences.

When I use a div wrapper, I’m adding an extra element to the DOM, which can sometimes interfere with the layout or CSS styles. For example, using extra div tags may mess up my Flexbox or Grid layouts by introducing unnecessary elements. React Fragments, on the other hand, allow me to group multiple elements without adding any extra nodes to the DOM. This keeps the structure clean and avoids layout issues.

Here’s an example with a div wrapper:

function MyComponent() {
  return (
    <div>
      <h1>Hello</h1>
      <p>Welcome to the app</p>
    </div>
  );
}

Here’s the same example with a React Fragment:

function MyComponent() {
  return (
    <>
      <h1>Hello</h1>
      <p>Welcome to the app</p>
    </>
  );
}

With a React Fragment (written as <>), no extra div is added to the DOM. This is more efficient when I don’t need the extra wrapper element. I use React Fragments when I want to group elements without affecting the DOM structure or styles, which is often the preferred approach for cleaner, more optimized code.

Explore: Props and State in React

22. What is the useCallback hook, and when should it be used?

The useCallback hook is a tool I use to memoize callback functions in React. Its main purpose is to prevent unnecessary re-creations of functions, which can help optimize performance in certain scenarios. In React, whenever a component re-renders, its functions are recreated, and this can sometimes cause performance issues, especially if those functions are passed down as props to child components that might re-render unnecessarily.

Here’s how I use useCallback:

const memoizedCallback = useCallback(() => {
  doSomething();
}, [dependency]);

In this example, useCallback only recreates the function when the dependency changes. If the dependency doesn’t change, React will reuse the previous version of the function, avoiding unnecessary re-renders in child components.

I typically use useCallback in performance-sensitive areas, such as:

  • When passing functions as props to child components that rely on React.memo for memoization.
  • Inside event handlers that don’t need to be recreated on every render.
  • With expensive calculations or operations that shouldn’t be re-executed unnecessarily.

That said, I don’t use useCallback all the time. It’s most useful when performance is a concern, especially in large applications where re-renders can slow things down. But overusing it in simpler components may add unnecessary complexity without significant performance benefits.

Explore: Creating a Sample Service in React JS

23. How would you handle conditional rendering in React?

Conditional rendering in React allows me to render different components or elements based on certain conditions. It’s a way for me to control what gets displayed to the user depending on the current state or props.

One simple method of conditional rendering is using if statements inside my component’s render function:

function MyComponent({ isLoggedIn }) {
  if (isLoggedIn) {
    return <h1>Welcome back!</h1>;
  } else {
    return <h1>Please log in</h1>;
  }
}

In this example, the component renders different headings based on the isLoggedIn prop. If the user is logged in, it displays a welcome message; otherwise, it prompts them to log in.

Another method I commonly use is the ternary operator, which is more concise and works well for simpler conditions:

function MyComponent({ isLoggedIn }) {
  return (
    <div>
      {isLoggedIn ? <h1>Welcome back!</h1> : <h1>Please log in</h1>}
    </div>
  );
}

I also use logical AND (&&) operators for rendering something only if a condition is true:

function MyComponent({ hasMessages }) {
  return (
    <div>
      {hasMessages && <h1>You have new messages!</h1>}
    </div>
  );
}

In this example, the message will only appear if hasMessages is true. Conditional rendering helps me create dynamic user interfaces, tailoring the UI based on user interactions or data changes.

Explore: Event Handling in Reactjs

24. Explain the concept of hydration in React and where it’s used.

Hydration in React refers to the process of taking server-rendered HTML and attaching React’s event handlers to it, making the page fully interactive. It’s a crucial step in Server-Side Rendering (SSR) where the initial HTML is generated on the server and sent to the browser, and then React takes over on the client side.

When using SSR, React generates the HTML on the server and sends it to the client as a fully rendered static page. Once the page loads in the browser, React runs a process called hydration, where it binds the existing HTML with the React components, attaching event listeners and updating the DOM where necessary.

Here’s how hydration is set up in React:

import { hydrateRoot } from 'react-dom/client';
hydrateRoot(document.getElementById('root'), <App />);

In this example, hydrateRoot is used instead of createRoot, as it’s designed specifically for hydrating server-rendered HTML. Hydration is used in SSR frameworks like Next.js, where I want to serve pre-rendered content to improve the performance and SEO of the application. By sending static HTML, the user can see content almost immediately while React takes care of interactivity in the background.

Read more: Arrays in Java interview Questions and Answers

25. How would you implement server-side rendering (SSR) in a React application, and what are its benefits?

To implement Server-Side Rendering (SSR) in a React application, I need to use a framework or tool like Next.js or set up my own Node.js server using react-dom/server. SSR involves rendering React components on the server and sending the fully rendered HTML to the client. This helps with performance and SEO, as the page content is available immediately to both users and search engines.

Here’s how I’d render a React app on the server using Node.js and react-dom/server:

import express from 'express';
import { renderToString } from 'react-dom/server';
import App from './App';

const app = express();

app.get('*', (req, res) => {
  const content = renderToString(<App />);
  const html = `
    <html>
      <head></head>
      <body>
        <div id="root">${content}</div>
        <script src="/bundle.js"></script>
      </body>
    </html>
  `;
  res.send(html);
});

app.listen(3000, () => console.log('Server is running on port 3000'));

In this example, I’m using Express to serve the React app. The renderToString function generates a string of HTML from my App component, and I send that string as part of the response to the client. Once the HTML is received on the client side, React will hydrate the app and make it interactive.

The main benefits of SSR include:

  • Improved SEO: Search engines can easily index the server-rendered content.
  • Faster initial load times: Users can see the content almost immediately, improving their experience on slow networks.
  • Better performance for large apps: By offloading the initial rendering to the server, the client device has less work to do initially.

Using SSR is especially beneficial for apps that require better SEO, like blogs or e-commerce websites, or for performance-critical applications where reducing the initial load time is important.

Conclusion

Mastering React JS at an advanced level, especially for a developer with 5 years of experience, requires a deep understanding of key concepts such as hooks, performance optimization, and state management. As React continues to evolve, knowing how to handle complex features like server-side rendering, memoization, and the context API is essential for building high-performance applications. These concepts not only help streamline the development process but also ensure that applications are efficient, scalable, and maintainable.

Being well-versed in conditional rendering, testing strategies, and advanced topics like hydration and React Fragments adds versatility to a developer’s skill set. It’s crucial to stay updated with new React features and best practices, as they allow you to write more robust and modern code. With a strong command over these concepts, developers can confidently face challenging React JS interview questions and excel in building dynamic, user-friendly applications that meet modern web standards.

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