Lifecycle Methods in React JS Interview Questions

Table Of Contents

• Lifecycle methods in React
• Class components and functional components
• Purpose of componentDidMount
• How can you prevent unnecessary re-renders
• What is componentDidCatch
• What are the potential pitfalls of using componentWillMount?
• How do you handle side effects in lifecycle methods?
• What are the best practices for using setState within lifecycle methods?
• How can you test lifecycle methods in your React components?
• How can you use the ref attribute in conjunction
• When should you clean up resources in componentWillUnmount?

React.js is a popular tool for building websites and apps. It uses “components” to create different parts of a page. Each component goes through several stages, such as being created, updated, and removed. These stages are called the component’s lifecycle. Lifecycle methods in React help control what happens at each stage. As a result, knowing these methods is important because they improve how React apps perform. This is why React JS interview questions often focus on lifecycle methods.

In interviews, you might be asked how to handle tasks like fetching data, cleaning up resources, or stopping unnecessary updates. For example, these tasks rely heavily on lifecycle methods in React. Although React now has “hooks” to manage these tasks, understanding older methods like componentDidMount, componentDidUpdate, and componentWillUnmount is still important. Therefore, preparing for React JS interview questions that cover both new and old methods will allow you to explain how to manage components effectively and showcase your skills.

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1. What are lifecycle methods in React, and why are they important?

Lifecycle methods in React are special functions that allow us to hook into different stages of a component’s lifecycle, such as when it is being created, updated, or removed from the DOM. These methods help us control the behavior of the component during these stages. For instance, we can fetch data from an API when the component first mounts or clean up any resources when the component unmounts. Understanding these lifecycle methods is crucial for optimizing the performance of React applications and ensuring they behave as expected.

They are important because they give us more control over how and when the React component interacts with the DOM. If we want our app to perform specific actions at particular moments in a component’s lifecycle, such as stopping a timer or saving user data, lifecycle methods help us handle that smoothly. Using these methods effectively ensures that our components are efficient and avoid unnecessary re-renders, which improves overall performance.

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

2. Can you explain the difference between class components and functional components regarding lifecycle methods?

In class components, lifecycle methods are built-in functions that React provides, such as componentDidMount, componentDidUpdate, and componentWillUnmount. These methods allow us to control what happens at each stage of the component’s life. For example, when a component mounts, we can use componentDidMount to fetch data from an API or interact with external systems. Class components rely heavily on these lifecycle methods to manage their behavior.

In functional components, we use React Hooks to achieve the same result as lifecycle methods in class components. Hooks like useEffect replace methods like componentDidMount, componentDidUpdate, and componentWillUnmount. For example, instead of using componentDidMount, we can write:

useEffect(() => {
  // This runs when the component mounts
  fetchData();
}, []);

The empty array ([]) ensures that this effect runs only once, similar to componentDidMount. Hooks provide a cleaner, more flexible way to manage the lifecycle of functional components.

3. What is the significance of the constructor method in a React component?

The constructor method in a React class component is the very first method that gets called when the component is created. It is used to set up the initial state of the component and bind any methods that need access to this. By defining the initial state in the constructor, we ensure that the component has the correct starting values for its state.

For example, in a simple counter app, the constructor could look like this:

constructor(props) {
  super(props);
  this.state = { count: 0 };
  this.incrementCount = this.incrementCount.bind(this);
}

In this example, I’ve initialized the count state and bound the incrementCount method to ensure this refers to the class instance. Without the constructor, our component wouldn’t know how to start and handle any data it needs at the beginning. Also, it’s important to note that in functional components, we don’t need a constructor as the useState hook takes care of setting up the initial state.

Explore: React JSX

4. What is the purpose of componentDidMount?

The componentDidMount method is called immediately after a component is inserted into the DOM. Its main purpose is to trigger any side effects or actions that we want to run after the component has rendered for the first time. For example, a common use case is to fetch data from an API right after the component mounts, so that the data can be displayed in the component.

For instance, I would write something like this:

componentDidMount() {
  fetch('https://api.example.com/data')
    .then(response => response.json())
    .then(data => this.setState({ data }));
}

In this example, I use componentDidMount to fetch data from an external API and update the component’s state with the result. This method is crucial for tasks like initial data loading, starting timers, or setting up subscriptions, which need to occur only once when the component first mounts.

Additionally, because componentDidMount runs after the initial render, it’s a good place to make DOM manipulations if necessary. However, I need to be careful not to trigger updates that will cause unnecessary re-renders, as that could affect performance.

Explore: Event Handling in Reactjs

5. What would you do if an API call fails in componentDidMount?

If an API call fails in componentDidMount, it’s essential to handle the error gracefully. The best approach is to use a catch block to manage any errors that occur during the fetch process. This allows me to display an error message to the user and prevent the app from crashing. For instance, I could modify the API call like this:

componentDidMount() {
  fetch('https://api.example.com/data')
    .then(response => {
      if (!response.ok) {
        throw new Error('Network response was not ok');
      }
      return response.json();
    })
    .then(data => this.setState({ data }))
    .catch(error => this.setState({ error: error.message }));
}

In this example, I’m catching errors and updating the component’s state with the error message. If an error occurs, I can display a user-friendly error message instead of breaking the app. This approach makes the app more resilient and improves the user experience by providing meaningful feedback when things go wrong.

Additionally, I can show a loading spinner while the data is being fetched and hide it once the request is complete. This gives users a better understanding of what’s happening while the data is loading and handles the case where the API call fails without leaving the UI in an unfinished state.

6. How does componentWillUnmount work, and why is it used?

componentWillUnmount is called right before a component is removed or “unmounted” from the DOM. Its main role is to clean up any side effects or resources that were set up during the component’s lifecycle. This method is essential for preventing memory leaks, especially when dealing with event listeners, timers, or subscriptions that were established in earlier lifecycle methods like componentDidMount.

For example, let’s say I start a timer in componentDidMount. I would use componentWillUnmount to clear that timer:

componentDidMount() {
  this.timerID = setInterval(() => this.tick(), 1000);
}

componentWillUnmount() {
  clearInterval(this.timerID);
}

In this code, componentWillUnmount ensures that the interval is cleared when the component is about to be removed from the DOM. Without this cleanup, the timer would continue running even after the component is gone, potentially causing performance issues. It’s also important for cleaning up any network requests or canceling subscriptions, ensuring that resources are properly freed when the component is no longer needed.

Explore: Form Handling in React JS

7. When would you use componentDidUpdate?

I would use componentDidUpdate whenever I need to perform an action after the component has been updated due to changes in props or state. This method is called after every re-render, except for the initial render. It’s useful for performing side effects that depend on the component’s updated state or props, such as making API calls, updating the DOM, or triggering animations.

For example, I might want to make an API call when a particular prop changes:

componentDidUpdate(prevProps) {
  if (this.props.userID !== prevProps.userID) {
    this.fetchUserData(this.props.userID);
  }
}

In this case, I’m checking if the userID prop has changed. If it has, I fetch new user data based on the updated userID. Without this check, the API call would trigger on every update, which would be inefficient and unnecessary.

Another scenario where componentDidUpdate is helpful is when we need to interact with the DOM in response to updated state. For example, I could use it to scroll to a specific element after new content has been rendered. However, it’s crucial to use this method carefully to avoid unnecessary re-renders, as it can lead to performance issues if not managed properly.

8. How can you prevent unnecessary re-renders using lifecycle methods?

To prevent unnecessary re-renders in a React component, I can use lifecycle methods like shouldComponentUpdate. This method allows me to control whether a component should re-render or not based on changes in state or props. By returning false in shouldComponentUpdate, I can block the re-rendering process, which helps improve performance, especially in large or complex applications.

Here’s a simple example:

shouldComponentUpdate(nextProps, nextState) {
  if (this.state.value === nextState.value) {
    return false; // Prevent re-render if state hasn't changed
  }
  return true; // Allow re-render for new state
}

In this example, I’m comparing the current state.value with the next state.value. If they are the same, I return false, which prevents the component from re-rendering unnecessarily. This is particularly useful in scenarios where the same data is being passed to the component multiple times.

In functional components, the React.memo function is often used for the same purpose. Memoization prevents re-renders if the props remain unchanged, effectively optimizing performance. However, whether in class components or functional components, I always need to ensure that I’m not blocking legitimate updates, as this could lead to bugs in my application.

Explore: React Router Interview Questions

9. What is the role of getDerivedStateFromProps?

getDerivedStateFromProps is a static lifecycle method in class components that allows the component to update its state based on changes in props before rendering. It is called right before the render method, and its purpose is to keep the component’s state in sync with its props. Since it’s a static method, it doesn’t have access to this, so the state must be returned explicitly from the method.

For example, if I want to update a component’s state when a prop changes, I can use getDerivedStateFromProps like this:

static getDerivedStateFromProps(nextProps, prevState) {
  if (nextProps.someProp !== prevState.someProp) {
    return { someProp: nextProps.someProp };
  }
  return null; // No state update if props are the same
}

In this code, I’m checking if the someProp value in the new props is different from the current state. If it is, I update the state with the new prop value. If not, I return null, indicating that no update is needed.

This method can be useful in scenarios where state needs to be recalculated or synchronized with incoming props. However, overusing it can lead to performance problems because it adds an additional layer of complexity to how state is managed. It’s generally recommended to use this method only when absolutely necessary, as React’s hooks and the newer functional component patterns often handle this kind of synchronization more efficiently.

Explore: React Hooks: Revolutionizing Functional Components

10. Can you describe a scenario where you might use shouldComponentUpdate?

shouldComponentUpdate is a powerful lifecycle method that I use to control whether a component should update or not. A common scenario where I would use this method is when performance is a concern, and I want to prevent unnecessary re-renders of a component. For instance, if the component receives new props frequently but the UI doesn’t need to change for every update, I can use shouldComponentUpdate to compare the current props with the incoming ones and decide whether to trigger a re-render.

Let’s take an example of a large list component that only needs to re-render when a specific item in the list changes:

shouldComponentUpdate(nextProps) {
  if (this.props.items !== nextProps.items) {
    return true; // Re-render if items prop changes
  }
  return false; // Skip re-render if items are the same
}

Here, I’m comparing the items prop to the nextProps.items. If they’re different, I allow the component to re-render. Otherwise, I block the re-render to optimize performance. This approach is especially helpful in applications where data updates frequently, but only small portions of the UI need to change.

By using shouldComponentUpdate effectively, I can significantly reduce the number of unnecessary re-renders, leading to better performance and a smoother user experience. However, I need to be careful when implementing this method, as blocking re-renders when they’re needed can lead to stale data or broken UI components. Therefore, it’s essential to carefully analyze when a component truly needs to update

11. How can you prevent unnecessary re-renders using lifecycle methods?

Preventing unnecessary re-renders is key to improving the performance of a React application. One way to achieve this in class components is by using the shouldComponentUpdate method. This lifecycle method gives us control over whether the component should re-render when new props or state are received. If we return false from this method, React skips the re-rendering process, saving computation time and improving performance.

For example, if I only want to re-render a component when a specific prop changes, I could write the following:

shouldComponentUpdate(nextProps, nextState) {
  if (this.props.value !== nextProps.value) {
    return true;  // Re-render only when 'value' changes
  }
  return false;  // Skip re-render for other changes
}

In functional components, I can use React.memo to prevent re-renders. It’s a higher-order component that only re-renders if the props have changed. This is useful when working with components that receive the same data repeatedly but don’t need to re-render every time.

Explore: Lifecycle Methods in React

12. What is the role of getDerivedStateFromProps?

getDerivedStateFromProps is a static lifecycle method that runs when a component is about to receive new props. It allows me to update the component’s state based on the incoming props before rendering happens. This method is especially useful in scenarios where the state depends on the props, and I need to synchronize them.

For instance, if I have a component that shows user data and the userID prop changes, I might use getDerivedStateFromProps to update the state with the new userID:

static getDerivedStateFromProps(nextProps, prevState) {
  if (nextProps.userID !== prevState.userID) {
    return { userID: nextProps.userID };
  }
  return null;  // No change in state if props are the same
}

In this example, I check if the userID prop has changed. If it has, I update the state with the new userID. If not, I return null, indicating no state change is necessary. This method ensures that the component stays in sync with its props, but it’s important to use it sparingly, as React Hooks now offer more flexible solutions for handling this kind of logic.

13. How do you use getSnapshotBeforeUpdate, and when would you need it?

getSnapshotBeforeUpdate is a lifecycle method that is called right before the DOM updates but after the render method. Its primary role is to capture some information from the DOM before it changes. This method is useful when we need to preserve the component’s state right before an update, such as the scroll position of a list or the current dimensions of an element.

For example, I might use getSnapshotBeforeUpdate to capture the scroll position of a chat window before new messages are added:

getSnapshotBeforeUpdate(prevProps, prevState) {
  if (prevProps.messages.length < this.props.messages.length) {
    return this.chatWindow.scrollHeight;
  }
  return null;
}

componentDidUpdate(prevProps, prevState, snapshot) {
  if (snapshot !== null) {
    this.chatWindow.scrollTop = this.chatWindow.scrollHeight - snapshot;
  }
}

In this code, I capture the scroll height of the chat window before the new messages are added and then adjust the scroll position after the update so the user doesn’t lose their place. getSnapshotBeforeUpdate is necessary when we need to perform DOM adjustments that rely on the pre-update state of the component.

Explore: Component Composition in React

14. What is componentDidCatch, and how does it differ from other lifecycle methods?

componentDidCatch is a lifecycle method used to handle errors in React components. It acts as a boundary for catching errors that occur during rendering, in lifecycle methods, or within the component tree of its child components. When an error is thrown, componentDidCatch is triggered, allowing me to log the error or display a fallback UI without crashing the entire application.

Unlike other lifecycle methods, which handle normal operations like rendering or updating components, componentDidCatch is specifically designed for error handling. For example:

componentDidCatch(error, info) {
  // Log the error to an external service
  logErrorToService(error, info);
  
  // Update state to show fallback UI
  this.setState({ hasError: true });
}

This method differs because it focuses on managing unexpected issues, providing a safe way to catch errors and recover gracefully. By using componentDidCatch, I can ensure the app doesn’t crash due to an error in one part of the component tree, and instead, display a user-friendly message while logging the issue for further debugging.

15. How do hooks in functional components relate to lifecycle methods in class components?

Hooks in functional components serve the same purpose as lifecycle methods in class components, but they are more flexible and easier to use. In class components, we have lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount to handle the different stages of a component’s lifecycle. In functional components, useEffect is the primary hook for managing side effects and lifecycle stages.

For example, useEffect can mimic componentDidMount by passing an empty dependency array ([]):

useEffect(() => {
  // Runs when the component mounts
  fetchData();
}, []);

Similarly, we can mimic componentWillUnmount by returning a cleanup function from useEffect:

useEffect(() => {
  const timer = setInterval(() => setTime(new Date()), 1000);

  // Cleanup function to run on unmount
  return () => clearInterval(timer);
}, []);

The key difference between hooks and lifecycle methods is that hooks combine multiple lifecycle functionalities into a single useEffect function, which allows for cleaner, more modular code. Hooks make it easier to handle complex component logic without the need to manage multiple lifecycle methods separately.

Explore: Conditional Rendering in React

16. What are the potential pitfalls of using componentWillMount?

componentWillMount is an older React lifecycle method that was used to run code right before a component mounts to the DOM. However, it has been deprecated in newer versions of React (starting from React 16.3) due to its unreliability and potential pitfalls. One of the key issues with componentWillMount is that it can cause race conditions when used to fetch data or perform asynchronous operations, as the component may not have mounted by the time these operations complete.

Additionally, any side effects triggered inside componentWillMount might run multiple times during server-side rendering (SSR), leading to inconsistent behavior. This makes it a poor choice for performing operations like fetching data or setting up subscriptions. Instead of componentWillMount, I should use componentDidMount for such tasks since it is more reliable and ensures that the component has fully rendered before any actions are taken.

In functional components, there is no equivalent of componentWillMount, and hooks like useEffect handle side effects in a much more predictable and reliable way. React also encourages the use of getDerivedStateFromProps and React.memo for updating state or avoiding unnecessary re-renders when needed.

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

17. How do you handle side effects in lifecycle methods?

Handling side effects in lifecycle methods requires careful consideration to ensure that they are managed correctly and efficiently. In class components, I typically use componentDidMount to handle side effects such as API calls, setting up subscriptions, or interacting with the DOM. For example, I might use componentDidMount to fetch data from a remote server:

componentDidMount() {
  fetch('https://api.example.com/data')
    .then(response => response.json())
    .then(data => this.setState({ data }));
}

When the component is unmounted, I need to ensure that any resources created by these side effects are properly cleaned up. This is where componentWillUnmount comes in. For example, if I set up a timer or event listener in componentDidMount, I should clean it up in componentWillUnmount to avoid memory leaks:

componentWillUnmount() {
  clearTimeout(this.timer);
  window.removeEventListener('resize', this.handleResize);
}

In functional components, I use hooks like useEffect to handle side effects. The advantage of useEffect is that it consolidates the logic for setting up and cleaning up side effects in one place, making it easier to manage:

useEffect(() => {
  const timer = setInterval(() => console.log('Tick'), 1000);
  
  return () => clearInterval(timer); // Cleanup on unmount
}, []);

By handling side effects properly, I ensure that my application remains stable and performant, even as components are mounted and unmounted repeatedly.

18. What would you do if an API call fails in componentDidMount?

If an API call fails in componentDidMount, the first step is to handle the error gracefully so that the application does not crash and the user receives meaningful feedback. I would implement error handling using a catch block to manage the failure and update the component’s state to reflect that an error occurred. This way, I can display an error message to the user instead of leaving the UI blank or broken.

For instance, here’s how I might handle a failed API call in componentDidMount:

componentDidMount() {
  fetch('https://api.example.com/data')
    .then(response => {
      if (!response.ok) {
        throw new Error('Network response was not ok');
      }
      return response.json();
    })
    .then(data => this.setState({ data }))
    .catch(error => this.setState({ error: error.message }));
}

In this example, if the API request fails (for instance, due to a network error or a 404 response), the error is caught, and the component’s state is updated with the error message. This allows me to display a user-friendly message on the UI, such as “Failed to load data, please try again later.”

Additionally, I would show a loading spinner while the API request is in progress, and hide it once the data has been fetched or an error has occurred. This improves the user experience by making it clear that the app is working to fetch data and gracefully handling the situation if something goes wrong.

Read more : Conditional Rendering in React JS

19. Can you describe how to manage timers in lifecycle methods?

Managing timers in lifecycle methods is crucial to ensure that they are properly set up and cleaned up. In class components, I typically set up a timer in the componentDidMount method, where I can start an interval or timeout to trigger specific actions. For example, I might set an interval to update the state every second:

componentDidMount() {
  this.timerID = setInterval(() => this.tick(), 1000);
}

The tick function might look something like this:

tick() {
  this.setState({ time: new Date().toLocaleTimeString() });
}

Once the component is unmounted, I must clean up the timer in componentWillUnmount to prevent memory leaks. If I forget to clean up the timer, it will keep running even after the component is no longer in the DOM, potentially causing performance issues.

componentWillUnmount() {
  clearInterval(this.timerID);
}

In functional components, managing timers is even easier with useEffect. I can set up the timer and return a cleanup function to clear it when the component unmounts:

useEffect(() => {
  const timerID = setInterval(() => setTime(new Date().toLocaleTimeString()), 1000);

  return () => clearInterval(timerID); // Cleanup on unmount
}, []);

By managing timers properly, I ensure that my components function as expected while avoiding memory leaks or performance problems.

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

20. What are the best practices for using setState within lifecycle methods?

Using setState within lifecycle methods requires caution to avoid performance issues and infinite re-renders. One key best practice is to avoid calling setState inside the render method, as it can lead to an infinite loop of re-rendering. Instead, I should use lifecycle methods like componentDidMount or componentDidUpdate to update the state when necessary.

For example, if I need to update the state after fetching data, I should call setState inside componentDidMount:

componentDidMount() {
  fetch('https://api.example.com/data')
    .then(response => response.json())
    .then(data => this.setState({ data }));
}

When using setState in componentDidUpdate, I must be careful to add conditions that prevent unnecessary updates. Without these conditions, I could trigger an infinite re-render. Here’s an example:

componentDidUpdate(prevProps) {
  if (this.props.value !== prevProps.value) {
    this.setState({ updatedValue: this.props.value });
  }
}

By comparing the current and previous props or state, I ensure that setState is only called when there’s an actual change. Another best practice is to use functional updates for setState when the new state depends on the previous state. This helps avoid race conditions:

this.setState(prevState => ({
  count: prevState.count + 1
}));

Following these best practices ensures that my use of setState within lifecycle methods is efficient, avoids unnecessary re-renders, and prevents potential bugs or performance bottlenecks.

Explore: Props and State in React

21. How can you test lifecycle methods in your React components?

Testing lifecycle methods in React components involves simulating the component’s mounting, updating, and unmounting phases to ensure they behave as expected. To achieve this, I often use testing libraries like Jest along with React Testing Library or Enzyme. These tools allow me to mock lifecycle methods and assert that they are called at the right times or produce the expected side effects.

For instance, if I want to test that componentDidMount correctly fetches data when a component is mounted, I could write a test like this using Jest and React Testing Library:

import { render, screen, waitFor } from '@testing-library/react';
import MyComponent from './MyComponent';

test('fetches data on componentDidMount', async () => {
  render(<MyComponent />);
  
  await waitFor(() => screen.getByText(/data loaded/i));
  
  expect(screen.getByText(/data loaded/i)).toBeInTheDocument();
});

This test renders the component and waits for the asynchronous data fetching to complete, checking that the fetched data is displayed correctly. By testing the behavior of lifecycle methods in this way, I can ensure that the component behaves as expected in a real-world scenario.

Similarly, I can use Enzyme to shallow render a component and simulate lifecycle events. For example, to test componentWillUnmount and ensure that cleanup occurs correctly, I might write a test that simulates unmounting:

import { shallow } from 'enzyme';
import MyComponent from './MyComponent';

test('cleans up on unmount', () => {
  const wrapper = shallow(<MyComponent />);
  wrapper.unmount();
  
  expect(wrapper.instance().cleanUpFunction).toHaveBeenCalled();
});

This type of test ensures that the component properly handles cleanup tasks, such as clearing timers or event listeners.

22. What are some common performance issues related to lifecycle methods?

Performance issues in React components often arise when lifecycle methods trigger unnecessary re-renders or execute costly operations. One common issue is improper use of shouldComponentUpdate, or neglecting to use it at all. By default, React re-renders components whenever state or props change, even if the new state or props don’t affect the component’s output. This can lead to performance bottlenecks, especially in large applications.

I can improve performance by implementing shouldComponentUpdate to control whether a component should re-render based on changes to its props or state. For instance, if I know that a component only needs to re-render when a specific prop changes, I can write:

shouldComponentUpdate(nextProps) {
  return this.props.value !== nextProps.value;
}

Another performance issue comes from running expensive computations inside lifecycle methods like componentDidUpdate or componentDidMount without considering optimization. If I’m making API calls or performing heavy computations within these methods, it can significantly slow down my app, especially if these operations are repeated multiple times unnecessarily.

To avoid such issues, I should optimize my component by memoizing results, using debouncing or throttling, and managing side effects efficiently. Using tools like React.memo in functional components can also help prevent unnecessary re-renders by only updating when specific props change.

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23. How can you use the ref attribute in conjunction with lifecycle methods?

The ref attribute in React allows me to directly access DOM elements or class component instances. It’s particularly useful in cases where I need to interact with the DOM, such as focusing on an input field, measuring an element’s dimensions, or triggering animations. Lifecycle methods like componentDidMount, componentDidUpdate, or componentWillUnmount can be used alongside ref to manage these interactions.

For example, if I want to focus on an input field when a component mounts, I can use ref in combination with componentDidMount:

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.inputRef = React.createRef();
  }

  componentDidMount() {
    this.inputRef.current.focus();
  }

  render() {
    return <input ref={this.inputRef} />;
  }
}

In this example, I create a ref using React.createRef() and assign it to the input element. Then, I use componentDidMount to call focus() on the input element, ensuring it gets focus when the component mounts.

In functional components, I use the useRef hook to achieve the same result:

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

  useEffect(() => {
    inputRef.current.focus();
  }, []);

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

This approach simplifies the code while maintaining the same functionality as class components.

24. How does the rendering phase differ from the committing phase in React’s lifecycle?

In React’s reconciliation process, the rendering phase and the committing phase are distinct parts of how React updates the DOM. The rendering phase is where React determines what changes need to be made to the virtual DOM. During this phase, React compares the current state of the virtual DOM with the new state generated by changes to props or state. However, no changes are applied to the actual DOM during this phase.

The committing phase, on the other hand, is when React takes the changes determined in the rendering phase and applies them to the actual DOM. This phase is where React updates the browser’s DOM and executes side effects such as running componentDidMount, componentDidUpdate, or componentWillUnmount.

In this split, the rendering phase is pure and synchronous—it doesn’t interact with the real DOM or cause side effects—while the committing phase handles the real-world effects, including DOM updates and invoking lifecycle methods that require interacting with external systems.

25. When should you clean up resources in componentWillUnmount?

I should always clean up resources in componentWillUnmount to prevent memory leaks and performance issues. Resources like timers, event listeners, network requests, or subscriptions can continue running even after the component is unmounted, causing unintended side effects and slowing down the application.

For example, if I set up an event listener in componentDidMount, I should remove it in componentWillUnmount:

componentDidMount() {
  window.addEventListener('resize', this.handleResize);
}

componentWillUnmount() {
  window.removeEventListener('resize', this.handleResize);
}

Similarly, I should clear any timers or intervals that are set up while the component is active:

componentDidMount() {
  this.timerID = setInterval(() => this.tick(), 1000);
}

componentWillUnmount() {
  clearInterval(this.timerID);
}

By cleaning up resources in componentWillUnmount, I ensure that my application runs efficiently and avoids unnecessary processes running in the background after a component has been removed from the DOM.

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Conclusion

As I keep learning React, I see that mastering lifecycle methods is a great start. It lays a strong foundation for using more advanced features, like hooks in functional components. Understanding when and how to use lifecycle methods improves the user experience. It also makes my code easier to read and maintain.

By improving my approach to lifecycle management, I feel more prepared. I can create high-quality and efficient applications. These applications will meet user needs effectively.