Ahmed Radwan

Posted on May 22, 2023 • Originally published at nerdleveltech.com

Mastering React Interview: Top Common Questions and Best Practices

#react #career #javascript #tutorial

1. Introduction:
- 1.1. Importance of React interviews
- 1.2. How to use this guide
2. Getting to Know the Basics
3. Components and Props: Your Building Blocks
4. Managing Data and Component Lifecycles
5. React Hooks: Enhancing Functional Components
6. React and Redux: State Management Made Easy
7. React Router and Navigation: Building Seamless User Experiences
8. Performance Optimization Techniques
9. Testing and Best Practices: Ensuring Quality
10. Let Us Answer Some Questions
11. Conclusion: Wrapping Up and Preparing for Your React Interview

1. Introduction:

Welcome to our comprehensive guide to help you excel in React interviews! In this article, we'll provide you with an in-depth understanding of key React concepts and best practices, while keeping it simple and engaging. Our aim is to help you navigate the React interview process with ease and confidence. Let's dive in!

1.1. Importance of React interviews

React has become one of the most popular and widely-used front-end libraries, thanks to its flexibility, performance, and ease of use. As a result, React developers are in high demand, and companies are always on the lookout for skilled professionals who can hit the ground running.

Acing a React interview is not just about showcasing your technical prowess, but also demonstrating your problem-solving skills and ability to adapt to new challenges. By preparing well for these interviews, you're not only increasing your chances of landing that dream job but also expanding your knowledge and becoming a better developer.

1.2. How to use this guide

This guide is designed to cover the most common topics you may encounter during a React interview. Each section will provide you with a concise yet detailed explanation of the topic, as well as real-world examples and best practices. To get the most out of this guide, we recommend the following approach:

Start by reading through each section to gain a general understanding of the concepts.
Pay close attention to the code examples, as they will help you visualize the concepts in practice.
Try out the code examples yourself to reinforce your understanding and gain hands-on experience.
After completing the guide, test your knowledge by attempting sample React interview questions.

Remember, practice makes perfect. Don't be afraid to explore additional resources and tutorials to deepen your understanding of React. With dedication and perseverance, you'll be well-prepared to tackle any React interview question with confidence. Now, let's begin our journey towards mastering the React interview!

2. Getting to Know the Basics

In this section, we'll explore the fundamentals of React, its key features and benefits, and how it compares to other popular frameworks. Don't worry; we'll keep things simple and easy to understand, with clear examples to help you grasp the concepts.

2.1. What is React?

React, developed by Facebook, is an open-source JavaScript library for building user interfaces (UI) in web applications. It allows developers to create reusable UI components and efficiently manage the way data is displayed and updated. React has gained massive popularity among developers and companies alike for its simplicity, flexibility, and performance.

2.2. Key features and benefits

Here are some of the key features and benefits of using React:

Component-based architecture: React promotes a modular approach by breaking down complex UIs into smaller, reusable components. This makes the code more manageable, scalable, and easier to maintain.
Virtual DOM: React uses a virtual representation of the actual DOM (Document Object Model) to track changes in the UI. When a component's state changes, React calculates the difference (called "diffing") between the virtual DOM and the actual DOM, then updates only the affected elements. This process, known as "reconciliation," improves performance by minimizing DOM manipulations.
Unidirectional data flow: In React, data flows in a single direction (from parent to child components) through props. This makes it easier to understand and debug the flow of data within the application.
JSX (JavaScript XML): React uses JSX, a syntax extension for JavaScript, to describe the structure and appearance of UI components. JSX combines HTML and JavaScript, making it more readable and easier to write.

function WelcomeMessage() {
  return <h1>Hello, world!</h1>;
}

2.3. React vs other popular frameworks

React is often compared to other popular frameworks and libraries, such as Angular and Vue.js. Let's see how React stacks up against them:

React vs Angular: Angular, developed by Google, is a full-fledged framework that includes built-in solutions for routing, state management, and form validation. React, on the other hand, is a library focused solely on the UI. While Angular enforces a strict structure and follows the Model-View-Controller (MVC) pattern, React offers more flexibility, allowing developers to choose their preferred tools and libraries. React's virtual DOM and lightweight nature often result in better performance compared to Angular.
React vs Vue.js: Vue.js is a progressive framework with a gentle learning curve, making it a popular choice for developers new to the world of front-end development. Like React, Vue.js also utilizes a virtual DOM and a component-based architecture. However, Vue.js provides a more flexible template syntax compared to JSX. React has a larger community, more resources, and wider adoption in the industry, while Vue.js is praised for its simplicity and ease of integration.

In summary, React's popularity, flexibility, and performance make it a strong choice for front-end development. While other frameworks have their own strengths, React continues to be a dominant force in the industry. By understanding the fundamentals of React, you're laying a solid foundation for your journey as a React developer.

3. Components and Props: Your Building Blocks

In this section, we'll explore the core concepts of React components and props, and how they help you create powerful and reusable UI elements. We'll keep things simple, with clear explanations and code examples to illustrate the concepts.

3.1. Understanding components

Components are the building blocks of any React application. They represent individual UI elements, such as buttons, forms, or lists, and can be reused throughout your application. Components can be combined and nested to create complex UIs while keeping the code clean and maintainable.

A React component can be defined as a simple JavaScript function that takes input (called "props") and returns a React element describing what should appear on the screen.

function WelcomeMessage(props) {
  return <h1>Hello, {props.name}!</h1>;
}

3.2. Functional vs class components

There are two ways to create components in React: functional components and class components.

Functional components: These are simple JavaScript functions that take props as input and return a React element. They are the most common type of components in modern React applications due to their simplicity and support for hooks.

function Greeting(props) {
  return <h1>Welcome, {props.name}!</h1>;
}

Class components: These are ES6 classes that extend React.Component. They include a render() method that returns a React element and can have additional methods and state.

class Greeting extends React.Component {
  render() {
    return <h1>Welcome, {this.props.name}!</h1>;
  }
}

While class components were once the standard, functional components are now preferred for their simplicity and the introduction of React hooks, which enable state and lifecycle features in functional components.

3.3. Props and their usage

Props (short for "properties") are the way data is passed from parent components to child components in React. They allow you to create dynamic and reusable components. Props are read-only, which means child components should not modify the props they receive.

Here's an example of passing props to a Greeting component:

function App() {
  return <Greeting name="John" />;
}

In the Greeting component, you can access the name prop using props.name:

function Greeting(props) {
  return <h1>Welcome, {props.name}!</h1>;
}

3.4. PropTypes and type checking

In a React application, it's essential to ensure that components receive the correct type of props. PropTypes is a built-in library that allows you to define the expected prop types for a component and display warnings during development if the types don't match.

To use PropTypes, first, import the library:

import PropTypes from 'prop-types';

Next, define the expected prop types for your component:

Greeting.propTypes = {
  name: PropTypes.string.isRequired,
};

In this example, we've specified that the name prop should be a required string. If the Greeting component receives a prop with an incorrect type, a warning will be displayed in the console during development.

4. Managing Data and Component Lifecycles

In this section, we'll explore the concepts of state and lifecycle methods in React, and how they help you manage your application's data and component behavior. We'll provide simple explanations and code examples to help you grasp these important concepts.

4.1. What is state?

State is an object that holds data relevant to a component. It allows components to have dynamic behavior by storing and updating the data displayed on the screen. State is private and fully controlled by the component it belongs to. When a component's state changes, React automatically re-renders the component to update the UI.

4.2. Handling state in class components

In class components, you can initialize state in the constructor and use the setState() method to update the state. Here's an example of a simple counter component using state in a class component:

class Counter extends React.Component {
  constructor(props) {
    super(props);
    this.state = { count: 0 };
  }

  increment() {
    this.setState({ count: this.state.count + 1 });
  }

  render() {
    return (
      <div>
        <h1>Count: {this.state.count}</h1>
        <button onClick={() => this.increment()}>Increment</button>
      </div>
    );
  }
}

4.3. Handling state in functional components

With the introduction of React hooks, managing state in functional components has become much easier. The useState() hook allows you to create state variables in functional components. Here's the same counter component using a functional component with the useState() hook:

import React, { useState } from 'react';

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

  function increment() {
    setCount(count + 1);
  }

  return (
    <div>
      <h1>Count: {count}</h1>
      <button onClick={increment}>Increment</button>
    </div>
  );
}

4.4. Common lifecycle methods

Lifecycle methods are special methods in class components that allow you to perform actions at different stages of a component's life, such as mounting, updating, or unmounting. Here are some common lifecycle methods:

componentDidMount: This method is called once the component has been rendered to the DOM. It's a good place to fetch data, set up timers, or perform other side effects.

componentDidMount() {
  // Fetch data, set up timers, or perform other side effects here
}

componentDidUpdate: This method is called whenever a component's props or state change. It allows you to perform side effects in response to updates, such as updating the DOM or fetching new data.

componentDidUpdate(prevProps, prevState) {
  // Perform side effects in response to component updates
}

componentWillUnmount: This method is called just before a component is removed from the DOM. It's a good place to clean up timers, cancel network requests, or remove event listeners.

componentWillUnmount() {
  // Clean up timers, network requests, or event listeners here
}

With React hooks, these lifecycle methods can be replaced by the useEffect() hook in functional components. For example, the componentDidMount and componentWillUnmount behavior can be achieved like this:

import React, { useEffect } from 'react';

function ExampleComponent() {
  useEffect(() => {
    // Perform side effects, similar to componentDidMount

    return () => {
      // Clean up, similar to componentWillUnmount
    };
  }, []);
// rest of the component
}

5. React Hooks: Enhancing Functional Components

In this section, we'll delve into the world of React hooks, a powerful feature that has transformed the way we write functional components. We'll cover the basics of hooks, their benefits, and the rules you need to follow when using them. As always, we'll provide clear explanations and code examples to help you grasp these important concepts.

5.1. What are hooks?

Hooks are a relatively new addition to React, introduced in version 16.8. They allow you to use state and other React features, like lifecycle methods, directly in functional components without having to convert them to class components. Hooks make it easier to reuse stateful logic between components, leading to cleaner and more maintainable code.

5.2. useState and useEffect

Two of the most commonly used hooks are useState and useEffect.

useState: This hook allows you to create state variables in functional components. It returns a pair: the current state value and a function to update it.

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

useEffect: This hook allows you to perform side effects in functional components, such as fetching data, setting up timers, or manipulating the DOM. It can replace lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount.

useEffect(() => {
  // Perform side effects here

  return () => {
    // Clean up when the component unmounts
  };
}, [dependencies]);

5.3. Custom hooks and their benefits

Custom hooks are user-defined hooks that allow you to encapsulate and reuse stateful logic between components. They follow the same naming convention as built-in hooks, starting with the word "use".

Creating a custom hook is as simple as extracting the logic from a functional component into a separate function. Here's an example of a custom hook for fetching data:

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

  useEffect(() => {
    fetch(url)
      .then((response) => response.json())
      .then((data) => {
        setData(data);
        setLoading(false);
      });
  }, [url]);

  return [data, loading];
}

By using custom hooks, you can:

Share stateful logic between components without duplicating code
Simplify complex components by separating concerns
Improve code readability and maintainability

5.4. Rules of hooks

There are two important rules you must follow when using hooks:

Only call hooks at the top level: Don't call hooks inside loops, conditions, or nested functions. This ensures that hooks are called in the same order on each render, allowing React to correctly preserve the state.
Only call hooks from React functions: Hooks should only be called from React functional components or custom hooks. Avoid calling hooks from regular JavaScript functions.

By adhering to these rules, you'll ensure that your hooks work as expected and maintain consistency in your code.

In conclusion, React hooks are a game-changer for functional components, enabling you to use state and lifecycle features without the need for class components.

6. React and Redux: State Management Made Easy

In this section, we'll discuss the popular state management library Redux and how it can be integrated with React applications. We'll cover the basics of Redux, its role in React applications, connecting components to the Redux store, and working with Redux middleware. As always, we'll provide simple explanations and code examples to help you understand these concepts.

6.1. What is Redux?

Redux is an open-source JavaScript library for managing the state of your application. It helps you maintain a predictable state container, making it easier to test and debug your applications. Redux is often used with React but can be utilized with other JavaScript frameworks as well.

6.2. The role of Redux in React applications

In a React application, managing state can become complex as your application grows. Redux provides a centralized store for the state of your application, making it easier to manage and share state across components. Redux enforces a unidirectional data flow and a strict update pattern, promoting consistency and predictability in your application's behavior.

6.3. Connecting React components to Redux store

To connect your React components to the Redux store, you'll need the react-redux library. The main components you'll work with are the Provider and connect function.

Provider: Wraps your application, making the Redux store available to all components.

import { Provider } from 'react-redux';
import store from './store';

ReactDOM.render(
  <Provider store={store}>
    <App />
  </Provider>,
  document.getElementById('root')
);

connect: A higher-order function that connects a React component to the Redux store, allowing it to access the store's state and dispatch actions.

import { connect } from 'react-redux';

function mapStateToProps(state) {
  return {
    counter: state.counter,
  };
}

function mapDispatchToProps(dispatch) {
  return {
    increment: () => dispatch({ type: 'INCREMENT' }),
  };
}

export default connect(mapStateToProps, mapDispatchToProps)(CounterComponent);

6.4. Redux middleware

Middleware in Redux is a way to extend the behavior of the store by adding custom logic in the middle of the dispatch process. Middleware can be used for various purposes, such as logging, handling asynchronous actions, or enforcing specific rules.

To use middleware in Redux, you'll need to apply it when creating the store:

import { createStore, applyMiddleware } from 'redux';
import thunk from 'redux-thunk';
import rootReducer from './reducers';

const store = createStore(rootReducer, applyMiddleware(thunk));

In this example, we're using the redux-thunk middleware, which allows you to write action creators that return functions instead of plain objects. This is useful for handling asynchronous actions, such as fetching data from an API.

function fetchPosts() {
  return (dispatch) => {
    dispatch(requestPosts());
    return fetch('https://jsonplaceholder.typicode.com/posts')
      .then((response) => response.json())
      .then((json) => dispatch(receivePosts(json)));
  };
}

By integrating Redux with your React applications, you can simplify state management, improve the predictability of your application's behavior, and make it easier to test and debug your code. Don't forget to practice working with Redux and React to solidify your understanding and become more comfortable with this powerful combination.

In this section, we'll discuss how to implement navigation in React applications using the popular library React Router. We'll cover the basics of configuring React Router, working with nested routes and route parameters, and handling redirects and protected routes. As always, we'll provide simple explanations and code examples to help you understand these concepts.

In single-page applications (SPAs) like those built with React, traditional navigation is replaced by client-side routing. React Router is a popular library for managing client-side navigation, allowing you to create seamless user experiences while maintaining the benefits of SPAs.

7.2. Configuring React Router

To get started with React Router, you'll need to install it and import the necessary components:

npm install react-router-dom

Then, import the components and configure your routes:

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

function App() {
  return (
    <Router>
      <nav>
        <ul>
          <li>
            <Link to="/">Home</Link>
          </li>
          <li>
            <Link to="/about">About</Link>
          </li>
          <li>
            <Link to="/contact">Contact</Link>
          </li>
        </ul>
      </nav>

      <Switch>
        <Route exact path="/" component={Home} />
        <Route path="/about" component={About} />
        <Route path="/contact" component={Contact} />
      </Switch>
    </Router>
  );
}

7.3. Nested routes and route parameters

React Router supports nested routes, allowing you to create more complex navigation structures. You can also use route parameters to capture values from the URL and pass them to your components:

import { Route, useParams } from 'react-router-dom';

function BlogPost() {
  const { postId } = useParams();
  // Fetch and display the post with the given postId
}

function Blog() {
  return (
    <>
      <Route exact path="/blog" component={BlogList} />
      <Route path="/blog/:postId" component={BlogPost} />
    </>
  );
}

7.4. Redirects and protected routes

Sometimes, you may want to redirect users based on certain conditions, like authentication status. React Router provides the Redirect component to handle this:

import { Route, Redirect } from 'react-router-dom';

function PrivateRoute({ component: Component, isAuthenticated, ...rest }) {
  return (
    <Route
      {...rest}
      render={(props) =>
        isAuthenticated ? (
          <Component {...props} />
        ) : (
          <Redirect to="/login" />
        )
      }
    />
  );
}

In this example, we've created a PrivateRoute component that checks the isAuthenticated prop. If the user is authenticated, they can access the route; otherwise, they are redirected to the login page.

8. Performance Optimization Techniques

In this section, we'll discuss various techniques to optimize the performance of your React applications. We'll cover topics like the React virtual DOM, PureComponent and React.memo, lazy loading and code splitting, and profiling to identify performance bottlenecks. As always, we'll provide simple explanations and code examples to help you understand these concepts.

8.1. Understanding the React virtual DOM

React uses a virtual DOM to improve the performance of your applications. The virtual DOM is an in-memory representation of the actual DOM, which allows React to calculate the difference (called the "diff") between the current and new DOM states. This way, React updates only the necessary parts of the actual DOM, reducing the amount of DOM manipulation and improving the performance.

8.2. PureComponent and React.memo

PureComponent and React.memo are two methods to optimize the performance of your components by preventing unnecessary re-renders.

PureComponent: PureComponent is a class component that only re-renders if its props or state have changed. It does a shallow comparison, which can help prevent unnecessary re-renders and improve performance.

import React, { PureComponent } from 'react';

class MyComponent extends PureComponent {
  render() {
    // Your component implementation
  }
}

React.memo: Similar to PureComponent, React.memo is a higher-order component for functional components that only re-renders if its props have changed. It also does a shallow comparison.

import React from 'react';

const MyComponent = React.memo(function MyComponent(props) {
  // Your component implementation
});

8.3. Lazy loading and code splitting

Lazy loading and code splitting are techniques to reduce the initial load time of your application by only loading the necessary parts of your application when needed.

Lazy loading: React.lazy() is a function that allows you to load components lazily, as they're needed. It works with dynamic imports, which return a Promise that resolves to a module containing a React component.

import React, { lazy, Suspense } from 'react';

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

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

Code splitting: Code splitting is the process of breaking up your application's code into smaller chunks, which can be loaded on-demand. This can be achieved using Webpack's dynamic imports or the React.lazy() function mentioned above.

8.4. Profiling and identifying performance bottlenecks

React provides a Profiler API to measure the performance of your application, allowing you to identify and address performance bottlenecks. The Profiler can be added to your component tree and will measure the rendering time and frequency of updates for the components within the tree.

import React, { Profiler } from 'react';

function onRenderCallback(
  id, // the "id" prop of the Profiler tree that has just committed
  phase, // either "mount" (if the tree just mounted) or "update" (if it re-rendered)
  actualDuration, // time spent rendering the committed update
  baseDuration, // estimated time to render the entire subtree without memoization
  startTime, // when React began rendering this update
  commitTime, // when React committed this update
  interactions // the Set of interactions belonging to this update
) {
  // Log the results
}

function App() {
  return (
    <Profiler id="MyComponent" onRender={onRenderCallback}>
      <MyComponent />
    </Profiler>
  );
}

9. Testing and Best Practices: Ensuring Quality

In this section, we'll discuss various testing strategies and best practices for your React applications. We'll cover topics like testing strategies, popular testing libraries and frameworks, mocking and snapshot testing, and tips for writing maintainable and scalable React code. As always, we'll provide simple explanations and code examples to help you understand these concepts.

9.1. Testing strategies for React applications

Testing is an essential part of developing robust React applications. A well-thought-out testing strategy can help you catch errors early and ensure that your application behaves as expected. Some common testing strategies for React applications include:

Unit testing: Testing individual components or functions in isolation.
Integration testing: Testing the interaction between multiple components or parts of your application.
End-to-end (E2E) testing: Testing your application as a whole, simulating real user interactions.

9.2. Popular testing libraries and frameworks

There are several popular libraries and frameworks available for testing React applications, including:

Jest: A popular testing framework that provides an extensive set of features for testing JavaScript and React applications.
React Testing Library: A library that encourages best practices for testing React components by focusing on their behavior and what the user sees.
Enzyme: A testing utility for React that makes it easier to test components by providing a shallow rendering API and other utilities.

9.3. Mocking and snapshot testing

Mocking and snapshot testing are two useful techniques for testing React components:

Mocking: Mocking is the process of creating fake objects or functions to replace real dependencies in your tests. This allows you to isolate the code being tested and control the behavior of external dependencies. Jest provides built-in support for mocking functions and modules.

import axios from 'axios';
import { fetchData } from './fetchData';

jest.mock('axios');

test('fetches data from the API', async () => {
  const data = { data: { result: 'test data' } };
  axios.get.mockResolvedValue(data);

  const result = await fetchData();

  expect(result).toEqual(data.data.result);
});

Snapshot testing: Snapshot testing compares the rendered output of a component to a stored snapshot, which serves as the expected output. If the output changes, the test will fail, ensuring that unintentional changes are caught early. Jest provides built-in support for snapshot testing.

import React from 'react';
import { render } from '@testing-library/react';
import MyComponent from './MyComponent';

test('matches the snapshot', () => {
  const { container } = render(<MyComponent />);
  expect(container.firstChild).toMatchSnapshot();
});

9.4. Tips for writing maintainable and scalable React code

To ensure that your React applications remain maintainable and scalable as they grow, follow these best practices:

Keep your components small and focused on a single responsibility.
Use functional components and hooks when possible, as they tend to be more concise and easier to test.
Use PropTypes to validate component props and catch errors early.
Write reusable and modular code, and avoid duplicating logic across components.
Keep your file and folder structure organized, and group related components and files together.

10. Let Us Answer Some Questions

In this section, we'll answer some common questions related to React, providing detailed information and code examples to help you understand the concepts more easily.

10.1. What are the differences between the real DOM vs virtual DOM and shadow DOM?

Real DOM: The real DOM (Document Object Model) is the actual representation of your web page in the browser's memory. Manipulating the real DOM is slow and performance-intensive.
Virtual DOM: The virtual DOM is an in-memory representation of the real DOM, used by React to calculate the difference (called "diff") between the current and new DOM states. This way, React updates only the necessary parts of the actual DOM, reducing the amount of DOM manipulation and improving performance.
Shadow DOM: The shadow DOM is a web standard that provides a way to create isolated, encapsulated DOM trees for web components. It helps to keep your components' markup, styles, and scripts separated from the main DOM, avoiding conflicts with other elements on the page.

10.2. What is the difference between an element and a component?

Element: An element is an immutable object that represents a DOM node. Elements are created using React's createElement function or JSX syntax.
Component: A component is a reusable piece of UI that can accept input (props) and return a React element. Components can be either functional (stateless) or class-based (stateful).

10.3. Can you write React without JSX?

Yes, you can write React without JSX. JSX is just syntactic sugar for React.createElement. Here's an example of a React component written without JSX:

import React from 'react';

function MyComponent() {
  return React.createElement('div', null, 'Hello, World!');
}

10.4. How to pass a value from a parent component to a child and vice versa?

To pass a value from a parent component to a child component, use props. To pass a value from a child component to a parent component, use a callback function. Here's an example:

import React, { useState } from 'react';

function Parent() {
  const [value, setValue] = useState('');

  return (
    <div>
      <Child value={value} onValueChange={setValue} />
    </div>
  );
}

function Child({ value, onValueChange }) {
  const handleChange = (e) => {
    onValueChange(e.target.value);
  };

  return (
    <input type="text" value={value} onChange={handleChange} />
  );
}

10.5. What is a higher-order component?

A higher-order component (HOC) is a function that takes a component and returns a new component with additional props or behavior. HOCs are a way to reuse component logic.

function withExtraProps(WrappedComponent, extraProps) {
  return function (props) {
    return <WrappedComponent {...props} {...extraProps} />;
  };
}

10.6. What is the difference between prop and state?

Prop: Props (short for properties) are the input to a component. They are passed from a parent component and are immutable within the component.
State: State is the internal data managed by a component. State can change over time and trigger a re-render of the component.

10.7. What is a component lifecycle?

A component lifecycle refers to the sequence of events that occur during a component's life in a React application, from its creation to its removal. Lifecycle methods in class components or hooks in functional components allow you to run code at specific points during the component's lifecycle, like mounting, updating , and unmounting.

10.8. What parameter does useEffect take in and when it runs?

useEffect is a hook that takes two parameters: a function (effect) and an optional dependency array. The effect function runs:

After the component mounts, when no dependencies are provided or when the dependencies have changed.
After the component updates if the dependencies have changed.
Before the component unmounts if a cleanup function is returned by the effect function.

Here's an example:

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

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

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

    return () => {
      document.title = 'React App';
    };
  }, [count]);

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

10.9. What is the difference between context API and prop-drilling?

Context API: The context API is a feature in React that allows you to share global data (like themes or user information) with multiple components without passing props through intermediate components. You create a context using React.createContext, then provide its value using a Provider component, and consume it in child components using the useContext hook or Consumer component.
Prop-drilling: Prop-drilling is a technique where you pass data through multiple levels of components in the component tree, even if some intermediate components don't need the data. It can lead to less maintainable and less efficient code. The context API can help mitigate prop-drilling by providing a way to share data without passing it through intermediate components.

11. Conclusion: Wrapping Up and Preparing for Your React Interview

In this final section, we will provide you with some tips on preparing for your React interview, discuss common pitfalls, and recommend additional resources to help you continue your learning journey.

11.1. Preparing for the Interview

Review the core concepts of React, such as components, state, props, hooks, and the component lifecycle.
Practice answering common React interview questions, like the ones covered in this article.
Build small projects or refactor existing ones to demonstrate your understanding of React best practices and patterns.
Familiarize yourself with related technologies, such as Redux, React Router, and popular testing libraries.

11.2. Common Pitfalls and How to Avoid Them

Not understanding the fundamentals: Ensure you have a strong foundation in JavaScript and React basics before diving into advanced concepts.
Over-engineering solutions: Focus on simplicity and readability. Avoid complex solutions when a simpler approach is sufficient.
Ignoring performance considerations: Be aware of the performance implications of your code and use optimization techniques, such as memoization, code splitting, and virtual DOM, when necessary.
Neglecting accessibility: Keep accessibility in mind and follow best practices, such as using semantic HTML and proper ARIA attributes.

11.3. Additional Resources and Next Steps

React Official Documentation: The official React documentation is an excellent resource for learning and staying up-to-date with React.
Redux Documentation: The official Redux documentation covers everything you need to know about using Redux with React.
React Router Documentation: The official React Router documentation provides guidance on using React Router for navigation in your React applications.
Create React App: A popular toolchain for creating and configuring new React projects with minimal setup.