Component Lifecycle

Every React component goes through a lifecycle: it’s created (mounted), updated, and eventually removed (unmounted). Understanding the component lifecycle helps you manage side effects, clean up resources, and optimize performance.

Lifecycle Phases

A component’s lifecycle can be divided into three main phases:

Mounting: Component is created and inserted into the DOM
Updating: Component re-renders due to changes in props or state
Unmounting: Component is removed from the DOM

Lifecycle in Function Components

Function components use Hooks to handle lifecycle events. The primary Hook for lifecycle management is useEffect.

Mounting and Updating

import { useState, useEffect } from 'react';

function UserProfile({ userId }) {
  const [user, setUser] = useState(null);
  const [loading, setLoading] = useState(true);

  // Runs after mount and whenever userId changes
  useEffect(() => {
    setLoading(true);
    
    fetch(`/api/users/${userId}`)
      .then(res => res.json())
      .then(data => {
        setUser(data);
        setLoading(false);
      });
  }, [userId]); // Dependency array

  if (loading) return <p>Loading...</p>;
  return <div>{user.name}</div>;
}

Effect Dependencies

The dependency array controls when the effect runs:

// Runs after every render
useEffect(() => {
  console.log('Rendered');
});

// Runs only once after mount
useEffect(() => {
  console.log('Component mounted');
}, []);

// Runs after mount and when dependencies change
useEffect(() => {
  console.log('count or name changed');
}, [count, name]);

Rules of Hooks: The useEffect Hook must be called at the top level of your component. Don’t call it inside conditions, loops, or nested functions. React relies on the order of Hook calls to track state correctly.

Cleanup (Unmounting)

Return a cleanup function from useEffect to run code when the component unmounts or before the effect runs again:

function Timer() {
  const [seconds, setSeconds] = useState(0);

  useEffect(() => {
    const interval = setInterval(() => {
      setSeconds(s => s + 1);
    }, 1000);

    // Cleanup function
    return () => {
      clearInterval(interval);
    };
  }, []); // Empty array means this runs once on mount

  return <div>Seconds: {seconds}</div>;
}

Common cleanup scenarios:

useEffect(() => {
  // Event listener
  function handleResize() {
    setWindowWidth(window.innerWidth);
  }
  
  window.addEventListener('resize', handleResize);
  
  return () => {
    window.removeEventListener('resize', handleResize);
  };
}, []);

useEffect(() => {
  // WebSocket connection
  const ws = new WebSocket('ws://localhost:8080');
  
  ws.onmessage = (event) => {
    setMessages(msgs => [...msgs, event.data]);
  };
  
  return () => {
    ws.close();
  };
}, []);

useEffect(() => {
  // Async operation that can be cancelled
  const controller = new AbortController();
  
  fetch('/api/data', { signal: controller.signal })
    .then(res => res.json())
    .then(data => setData(data))
    .catch(err => {
      if (err.name !== 'AbortError') {
        console.error(err);
      }
    });
  
  return () => {
    controller.abort();
  };
}, []);

useLayoutEffect

useLayoutEffect is similar to useEffect but fires synchronously after all DOM mutations, before the browser paints:

import { useLayoutEffect, useRef } from 'react';

function Tooltip() {
  const ref = useRef(null);
  
  useLayoutEffect(() => {
    // Measure DOM and update position before paint
    const { height } = ref.current.getBoundingClientRect();
    ref.current.style.marginTop = `-${height}px`;
  }, []);
  
  return <div ref={ref}>Tooltip content</div>;
}

When to use useLayoutEffect: Use it when you need to read layout information (like scroll position or element size) and synchronously make changes before the browser paints. For most cases, useEffect is preferred.

useInsertionEffect

useInsertionEffect fires before all DOM mutations. It’s primarily used by CSS-in-JS libraries to inject styles:

import { useInsertionEffect } from 'react';

function useCSS(rule) {
  useInsertionEffect(() => {
    // Inject CSS before DOM mutations
    const style = document.createElement('style');
    style.textContent = rule;
    document.head.appendChild(style);
    
    return () => {
      document.head.removeChild(style);
    };
  }, [rule]);
}

According to React’s source code in ReactHooks.js, the effect timing order is:

useInsertionEffect - Before DOM mutations
useLayoutEffect - After DOM mutations, before paint
useEffect - After paint

Lifecycle in Class Components

Class components have explicit lifecycle methods. While function components with Hooks are now preferred, understanding class lifecycle methods is useful for maintaining existing code.

Mounting

Called when a component is created and inserted into the DOM:

import { Component } from 'react';

class UserProfile extends Component {
  constructor(props) {
    super(props);
    // Initialize state
    this.state = {
      user: null,
      loading: true
    };
  }

  componentDidMount() {
    // Called after component is mounted
    // Perfect for: API calls, subscriptions, timers
    fetch(`/api/users/${this.props.userId}`)
      .then(res => res.json())
      .then(user => {
        this.setState({ user, loading: false });
      });
  }

  render() {
    if (this.state.loading) return <p>Loading...</p>;
    return <div>{this.state.user.name}</div>;
  }
}

Updating

Called when props or state change:

class UserProfile extends Component {
  componentDidUpdate(prevProps, prevState) {
    // Called after component updates
    // Compare previous and current props/state
    if (this.props.userId !== prevProps.userId) {
      this.loadUser(this.props.userId);
    }
  }

  shouldComponentUpdate(nextProps, nextState) {
    // Return false to prevent unnecessary re-renders
    // PureComponent does this automatically with shallow comparison
    return (
      nextProps.userId !== this.props.userId ||
      nextState.user !== this.state.user
    );
  }

  render() {
    return <div>{this.state.user?.name}</div>;
  }
}

According to the React source code in ReactBaseClasses.js, forceUpdate() will not invoke shouldComponentUpdate, but it will invoke componentWillUpdate and componentDidUpdate.

Unmounting

Called when a component is removed from the DOM:

class Timer extends Component {
  componentDidMount() {
    this.interval = setInterval(() => {
      this.setState({ seconds: this.state.seconds + 1 });
    }, 1000);
  }

  componentWillUnmount() {
    // Clean up before component is unmounted
    // Cancel timers, subscriptions, network requests
    clearInterval(this.interval);
  }

  render() {
    return <div>Seconds: {this.state.seconds}</div>;
  }
}

Error Boundaries

Class components can catch errors in child components using error boundaries:

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

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

  componentDidCatch(error, errorInfo) {
    // Log error to error reporting service
    console.error('Error caught:', error, errorInfo);
  }

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

    return this.props.children;
  }
}

// Usage
<ErrorBoundary>
  <App />
</ErrorBoundary>

Error boundaries do not catch errors in:

Event handlers (use try/catch instead)
Asynchronous code (setTimeout, promises)
Server-side rendering
Errors thrown in the error boundary itself

Function vs Class Lifecycle Comparison

Function Component
Class Component

import { useState, useEffect } from 'react';

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

// componentDidMount + componentDidUpdate
useEffect(() => {
const controller = new AbortController();

fetch(`/api/data/${id}`, { signal: controller.signal })
  .then(res => res.json())
  .then(setData);

// componentWillUnmount
return () => controller.abort();
}, [id]);

return <div>{data?.name}</div>;
}

import { Component } from 'react';

class DataFetcher extends Component {
constructor(props) {
super(props);
this.state = { data: null };
this.controller = null;
}

componentDidMount() {
this.loadData();
}

componentDidUpdate(prevProps) {
if (prevProps.id !== this.props.id) {
  this.loadData();
}
}

componentWillUnmount() {
if (this.controller) {
  this.controller.abort();
}
}

loadData() {
this.controller = new AbortController();

fetch(`/api/data/${this.props.id}`, {
  signal: this.controller.signal
})
  .then(res => res.json())
  .then(data => this.setState({ data }));
}

render() {
return <div>{this.state.data?.name}</div>;
}
}

Common Lifecycle Patterns

Fetching Data

function UserList() {
  const [users, setUsers] = useState([]);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

  useEffect(() => {
    fetch('/api/users')
      .then(res => res.json())
      .then(data => {
        setUsers(data);
        setLoading(false);
      })
      .catch(err => {
        setError(err);
        setLoading(false);
      });
  }, []);

  if (loading) return <p>Loading...</p>;
  if (error) return <p>Error: {error.message}</p>;
  return <ul>{users.map(u => <li key={u.id}>{u.name}</li>)}</ul>;
}

Subscribing to External Data

function ChatRoom({ roomId }) {
  const [messages, setMessages] = useState([]);

  useEffect(() => {
    const subscription = chatAPI.subscribe(roomId, (message) => {
      setMessages(msgs => [...msgs, message]);
    });

    return () => {
      subscription.unsubscribe();
    };
  }, [roomId]);

  return (
    <div>
      {messages.map((msg, i) => <p key={i}>{msg}</p>)}
    </div>
  );
}

Derived State

function SearchResults({ query }) {
  const [items, setItems] = useState([]);
  const [filteredItems, setFilteredItems] = useState([]);

  // Instead of using state for derived values, calculate them during render
  const filteredItems = items.filter(item =>
    item.name.toLowerCase().includes(query.toLowerCase())
  );

  return (
    <ul>
      {filteredItems.map(item => <li key={item.id}>{item.name}</li>)}
    </ul>
  );
}

If a value can be calculated from props or state, don’t store it in state. Calculate it during render instead. This prevents synchronization bugs.

Getting Started

Core Concepts

Building UIs

Advanced Guides

Performance

Component Lifecycle

Lifecycle Phases

Lifecycle in Function Components

Mounting and Updating

Effect Dependencies

Cleanup (Unmounting)

useLayoutEffect

useInsertionEffect

Lifecycle in Class Components

Mounting

Updating

Unmounting

Error Boundaries

Function vs Class Lifecycle Comparison

Common Lifecycle Patterns

Fetching Data

Subscribing to External Data

Derived State

Next Steps

Hooks Overview

Rendering

Build docs developers (and LLMs) love

Getting Started

Core Concepts

Building UIs

Advanced Guides

Performance

​Lifecycle Phases

​Lifecycle in Function Components

​Mounting and Updating

​Effect Dependencies

​Cleanup (Unmounting)

​useLayoutEffect

​useInsertionEffect

​Lifecycle in Class Components

​Mounting

​Updating

​Unmounting

​Error Boundaries

​Function vs Class Lifecycle Comparison

​Common Lifecycle Patterns

​Fetching Data

​Subscribing to External Data

​Derived State

​Next Steps

Hooks Overview

Rendering

Build docs developers (and LLMs) love

Lifecycle Phases

Lifecycle in Function Components

Mounting and Updating

Effect Dependencies

Cleanup (Unmounting)

useLayoutEffect

useInsertionEffect

Lifecycle in Class Components

Mounting

Updating

Unmounting

Error Boundaries

Function vs Class Lifecycle Comparison

Common Lifecycle Patterns

Fetching Data

Subscribing to External Data

Derived State

Next Steps