React Component Lifecycle Methods Complete Guide

1. What are the primary phases in a React component's lifecycle?

Answer: React components go through three primary phases:

• Mounting Phase: When React elements are inserted into the DOM.
• Updating Phase: When re-rendering because of a change in props or state.
• Unmounting Phase: When a React element is being removed from the DOM.
• In addition to these, there’s also an Error Handling Phase that deals with exceptions in the component tree.

2. List all the lifecycle methods available in class-based components.

Answer: Below are the common lifecycle methods of class-based components in React (as of React 18):

• Mounting:

  • constructor(props)
  • static getDerivedStateFromProps(props, state)
  • render()
  • componentDidMount()

• Updating:

  • static getDerivedStateFromProps(props, state)
  • shouldComponentUpdate(nextProps, nextState) [Deprecated in StrictMode in React 18]
  • render()
  • getSnapshotBeforeUpdate(prevProps, prevState)
  • componentDidUpdate(prevProps, prevState, snapshot)

• Unmounting:

  • componentWillUnmount()

• Error Handling:

  • static getDerivedStateFromError(error)
  • componentDidCatch(error, info)

3. What is the purpose of the constructor method in React?

Answer: The constructor method is used for initializing the state of a component and binding methods to the component instance. It is called before the component is mounted.

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

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

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

4. Explain getDerivedStateFromProps method and how it should be used.

Answer: getDerivedStateFromProps is a static method that returns an object to update the state based on props or null if no changes to state are needed. It is used when the state depends on incoming props.

class Welcome extends React.Component {
  static getDerivedStateFromProps(nextProps, prevState) {
    // Returns object for state updates
    return { greeting: nextProps.name };
  }

  constructor(props) {
    super(props);
    this.state = { greeting: '' };
  }

  render() {
    return (
      <div>
        <h1>Hello, {this.state.greeting}!</h1>
      </div>
    );
  }
}

5. When should componentDidMount be used?

Answer: componentDidMount is used after the component output has been rendered to the DOM. It is a great place to instantiate network requests, subscriptions, set up timers, or perform other side effects.

class Timer extends React.Component {
  constructor(props) {
    super(props);
    this.state = { time: new Date().toLocaleTimeString() };
  }

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

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

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

  render() {
    return (
      <div>
        <p>The time now is {this.state.time}.</p>
      </div>
    );
  }
}

6. What is the purpose of the componentDidUpdate method?

Answer: componentDidUpdate is invoked immediately after updating occurs. This method can’t have side effects except asynchronously in componentDidCatch or using setState. It provides access to the previous props and state.

class Counter extends React.Component {
  componentDidUpdate(prevProps, prevState) {
    if (prevState.count !== this.state.count) {
      console.log('Counter updated.');
    }
  }

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

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

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

7. How does getSnapshotBeforeUpdate work in React?

Answer: getSnapshotBeforeUpdate is called right before the most recently rendered output is committed to the DOM. It is used for reading the DOM before a potentially destructive update. Return a value which will be passed as a third argument to componentDidUpdate.

class ScrollingList extends React.Component {
  listRef = React.createRef();

  getSnapshotBeforeUpdate(prevProps, prevState) {
    // Are we adding new items to the list?
    // Capture the scroll position so we can adjust it after the update.
    if (prevProps.list.length < this.props.list.length) {
      const list = this.listRef.current;
      return list.scrollHeight - list.scrollTop;
    }
    return null;
  }

  componentDidUpdate(prevProps, prevState, snapshot) {
    if (snapshot !== null) {
      const list = this.listRef.current;
      list.scrollTop = list.scrollHeight - snapshot;
    }
  }

  render() {
    return (
      <ul ref={this.listRef}>
        {this.props.list.map(item => <li key={item.id}>{item.text}</li>)}
      </ul>
    );
  }
}

8. What is the significance of componentWillUnmount in React?

Answer: componentWillUnmount is used for cleanup just before a component is unmounted and destroyed. It can clear subscriptions, timers, and cancel network requests.

class WeatherFetcher extends React.Component {
  componentDidMount() {
    this.timerID = setInterval(() => this.fetchWeather(), 1000);
  }

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

  fetchWeather() {
    // Fetch weather...
  }

  render() {
    return <div>Fetching Weather...</div>;
  }
}

9. Describe the static getDerivedStateFromError and componentDidCatch methods.

Answer: These two methods are used for catching errors in descendant components (similar to a JavaScript try-catch block).

• static getDerivedStateFromError(error): It is used to update the state so that the next render shows an error message instead of the crashed component.
• componentDidCatch(error, info): It allows you to log the error information (which includes the name of the component that crashed and where in the tree did the crash occur).

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

  static getDerivedStateFromError(error) {
    // Update state so that next render displays fallback UI.
    return { hasError: true };
  }

  componentDidCatch(error, info) {
    // Log the error information
    console.log(info.componentStack);
  }

  render() {
    if (this.state.hasError) {
      // You can render any custom fallback UI
      return <h1>Oops! Something went wrong.</h1>;
    }
    return this.props.children;
  }
}

class BuggyComponent extends React.Component {
  render() {
    throw new Error('I crashed!');
  	return <h1>I am buggy</h1>;
  }
}

function App() {
  return (
    <ErrorBoundary>
      <BuggyComponent />
    </ErrorBoundary>
  );
}

10. Why were methods like componentWillReceiveProps and componentWillUpdate deprecated?

Answer: According to the official React documentation, componentWillReceiveProps and componentWillUpdate were prone to bugs and unnecessary invocations in some edge cases. These methods don't always fire as expected when dealing with async rendering which may cause stale props and states issues leading to inconsistent UIs.

To handle prop changes safely, developers are encouraged to use:

• static getDerivedStateFromProps
• The useEffect hook in functional components

For optimizing updates, developers can use:

• shouldComponentUpdate
• React.memo wrapped around functional components
• useMemo, useCallback

These new mechanisms provide more predictable control over updates in a component’s lifecycle without the pitfalls of the old ones.