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What are React component lifecycle methods?
Lifecycle methods are optional methods on class components that React calls at particular points in a component’s existence. A class needs only render() to describe its UI; lifecycle methods add behavior around that rendering, such as subscribing to a service or releasing a resource.
Keep render() pure: it should calculate UI from props, state, and context, not start network requests, change external systems, or interact with browser APIs. Use lifecycle methods for side effects after React has committed the relevant UI work.
What is the order of lifecycle methods in React?
The sequence depends on whether the component is mounting, updating, being removed, or handling an error. The table focuses on the methods most relevant to ordinary class-component work.
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| Method | When it runs | Typical role and caution |
|---|---|---|
constructor(props) |
Before the component mounts. | Initialize state or bind methods in older patterns. Do not start subscriptions or other side effects here; modern class fields often eliminate the need for a constructor. |
static getDerivedStateFromProps(props, state) |
Before render() on initial mount and later renders. |
Rarely derive state from props. Consider simpler controlled or uncontrolled component designs, or memoization, first. |
render() |
Whenever React needs to calculate the component’s UI. | Return UI as a pure calculation. React may call it more than once, so do not put side effects here. |
componentDidMount() |
After the component is added to the screen. | Start data fetching or subscriptions, or interact with DOM nodes. If setup depends on changing inputs, handle updates and cleanup too. |
shouldComponentUpdate(nextProps, nextState) |
Before React renders an update. | Optional rendering optimization. Returning false suppresses componentDidUpdate() and getSnapshotBeforeUpdate(); use only when the comparison is correct. |
getSnapshotBeforeUpdate(prevProps, prevState) |
Immediately before React updates the DOM. | Capture information, such as scroll position, that the DOM update could otherwise change. Its return value is passed to componentDidUpdate(). |
componentDidUpdate(prevProps, prevState, snapshot) |
After a re-render caused by changed props or state; not after the initial render. | Synchronize work when relevant inputs change. Compare previous and current values before acting or calling setState(). |
componentWillUnmount() |
Just before the component is removed. | Clean up subscriptions and other work started earlier. |
static getDerivedStateFromError(error) and componentDidCatch(error, info) |
When handling errors in descendants. | Class error boundaries can update state to show fallback UI and report error details. |
This is a guide to the relevant phases, not a promise that every method runs on every render. For example, componentDidUpdate() does not run on the first render, and a shouldComponentUpdate() result of false can prevent it from running for an update.
How do componentDidMount, componentDidUpdate, and componentWillUnmount work together?
Use the three methods as a setup–synchronize–cleanup pattern. The example below connects to a chat room when the component appears, switches connections if roomId changes, and closes the active connection before removal.
class ChatRoom extends Component {
componentDidMount() {
this.connect(this.props.roomId);
}
componentDidUpdate(prevProps) {
if (this.props.roomId !== prevProps.roomId) {
this.disconnect();
this.connect(this.props.roomId);
}
}
componentWillUnmount() {
this.disconnect();
}
render() {
return <h1>Room {this.props.roomId}</h1>;
}
}This is a schematic example: a real component must implement connect() and disconnect() using its actual service API. The important detail is the comparison in componentDidUpdate(). It avoids reconnecting when an unrelated update occurs. Likewise, if you call setState() from componentDidUpdate(), guard it with a meaningful comparison; an unguarded state change can cause a render loop and extra work.
componentDidMount versus componentDidUpdate
componentDidMount() runs after the first appearance on screen, so it is suited to initial setup. componentDidUpdate() runs after later updates, so it is suited to synchronizing with changed props or state. It receives the previous props and state for comparison, and an optional snapshot returned by getSnapshotBeforeUpdate().
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When to use componentWillUnmount
Use componentWillUnmount() to reverse or release setup performed earlier: unsubscribe, close a connection, or otherwise stop ongoing work. Matching cleanup to setup prevents a component that is gone from leaving work behind.
What is getSnapshotBeforeUpdate for?
Use getSnapshotBeforeUpdate() for the uncommon case where you must read information from the DOM immediately before React changes it. For example, a component might need to capture a scroll position that would be lost in the update, then use the returned value in componentDidUpdate(). It is class-specific in the current React reference and has no direct function-component equivalent.
Are componentWillMount and componentWillReceiveProps deprecated?
The legacy pre-render methods componentWillMount, componentWillReceiveProps, and componentWillUpdate were renamed with an UNSAFE_ prefix. They are historical APIs, not recommended for new code. Choose a replacement based on what the code needs to do: initialize state, perform post-mount setup, synchronize after an update, or capture pre-update DOM information.
How do React lifecycle methods map to useEffect?
In many cases, the work spread across componentDidMount(), componentDidUpdate(), and componentWillUnmount() can be expressed with useEffect() in a function component. The mapping is useful as a migration aid, but it is not a one-to-one conceptual model: React recommends thinking about each Effect as its own synchronization process. When work must run before the browser paints, useLayoutEffect() is the closer option. Not every class lifecycle method has a direct Hook equivalent.
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What should you know about lifecycle timing in Strict Mode?
In development, React Strict Mode may call componentDidMount(), then componentWillUnmount(), then componentDidMount() again. This helps reveal setup that lacks complete cleanup; it does not mean production mounts always happen twice. Make setup and cleanup safe to repeat rather than relying on a single development call.
When should you use a class component?
React continues to support class components, and their lifecycle methods remain relevant when maintaining or extending existing class-based code. React recommends function components for new components. Class components are also the documented way to implement error boundaries: use static getDerivedStateFromError() and componentDidCatch() to handle errors in descendant rendering and show fallback UI.
For the API details, see React’s Component reference and its explanation of the lifecycle of reactive Effects.
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