React.Component

Components let you split the UI into independent, reusable pieces, and think about each piece in isolation. React.Component is provided by React.

Overview

React.Component is an abstract base class, so it rarely makes sense to refer to React.Component directly. Instead, you will typically subclass it, and define at least a render() method.

Normally you would define a React component as a plain JavaScript class:

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

If you don’t use ES6 yet, you may use the create-react-class module instead. Take a look at Using React without ES6 to learn more.

The Component Lifecycle

Each component has several “lifecycle methods” that you can override to run code at particular times in the process. Methods prefixed with will are called right before something happens, and methods prefixed with did are called right after something happens.

Mounting

These methods are called when an instance of a component is being created and inserted into the DOM:

• constructor()
• componentWillMount()
• render()
• componentDidMount()

Updating

An update can be caused by changes to props or state. These methods are called when a component is being re-rendered:

• componentWillReceiveProps()
• shouldComponentUpdate()
• componentWillUpdate()
• render()
• componentDidUpdate()

Unmounting

This method is called when a component is being removed from the DOM:

• componentWillUnmount()

Error Handling

This method is called when there is an error during rendering, in a lifecycle method, or in the constructor of any child component.

• componentDidCatch()

Other APIs

Each component also provides some other APIs:

• setState()
• forceUpdate()

Class Properties

• defaultProps
• displayName

Instance Properties

• props
• state

Reference

render()

render()

The render() method is required.

When called, it should examine this.props and this.state and return one of the following types:

• React elements. Typically created via JSX. An element can either be a representation of a native DOM component (<div />), or a user-defined composite component (<MyComponent />).
• String and numbers. These are rendered as text nodes in the DOM.
• Portals. Created with ReactDOM.createPortal.
• null. Renders nothing.
• Booleans. Render nothing. (Mostly exists to support return test && <Child /> pattern, where test is boolean.)

When returning null or false, ReactDOM.findDOMNode(this) will return null.

The render() function should be pure, meaning that it does not modify component state, it returns the same result each time it’s invoked, and it does not directly interact with the browser. If you need to interact with the browser, perform your work in componentDidMount() or the other lifecycle methods instead. Keeping render() pure makes components easier to think about.

Note

render() will not be invoked if shouldComponentUpdate() returns false.

Fragments

You can also return multiple items from render() using an array:

render() {
  return [
    <li key="A">First item</li>,
    <li key="B">Second item</li>,
    <li key="C">Third item</li>,
  ];
}

Note:

Don’t forget to add keys to elements in a fragment to avoid the key warning.

constructor()

constructor(props)

The constructor for a React component is called before it is mounted. When implementing the constructor for a React.Component subclass, you should call super(props) before any other statement. Otherwise, this.props will be undefined in the constructor, which can lead to bugs.

Avoid introducing any side-effects or subscriptions in the constructor. For those use cases, use componentDidMount() instead.

The constructor is the right place to initialize state. To do so, just assign an object to this.state; don’t try to call setState() from the constructor. The constructor is also often used to bind event handlers to the class instance.

If you don’t initialize state and you don’t bind methods, you don’t need to implement a constructor for your React component.

In rare cases, it’s okay to initialize state based on props. This effectively “forks” the props and sets the state with the initial props. Here’s an example of a valid React.Component subclass constructor:

constructor(props) {
  super(props);
  this.state = {
    color: props.initialColor
  };
}

Beware of this pattern, as state won’t be up-to-date with any props update. Instead of syncing props to state, you often want to lift the state up instead.

If you “fork” props by using them for state, you might also want to implement componentWillReceiveProps(nextProps) to keep the state up-to-date with them. But lifting state up is often easier and less bug-prone.

componentWillMount()

componentWillMount()

componentWillMount() is invoked immediately before mounting occurs. It is called before render(), therefore calling setState() synchronously in this method will not trigger an extra rendering. Generally, we recommend using the constructor() instead.

Avoid introducing any side-effects or subscriptions in this method. For those use cases, use componentDidMount() instead.

This is the only lifecycle hook called on server rendering.

componentDidMount()

componentDidMount()

componentDidMount() is invoked immediately after a component is mounted. Initialization that requires DOM nodes should go here. If you need to load data from a remote endpoint, this is a good place to instantiate the network request.

This method is a good place to set up any subscriptions. If you do that, don’t forget to unsubscribe in componentWillUnmount().

Calling setState() in this method will trigger an extra rendering, but it will happen before the browser updates the screen. This guarantees that even though the render() will be called twice in this case, the user won’t see the intermediate state. Use this pattern with caution because it often causes performance issues. It can, however, be necessary for cases like modals and tooltips when you need to measure a DOM node before rendering something that depends on its size or position.

componentWillReceiveProps()

componentWillReceiveProps(nextProps)

componentWillReceiveProps() is invoked before a mounted component receives new props. If you need to update the state in response to prop changes (for example, to reset it), you may compare this.props and nextProps and perform state transitions using this.setState() in this method.

Note that React may call this method even if the props have not changed, so make sure to compare the current and next values if you only want to handle changes. This may occur when the parent component causes your component to re-render.

React doesn’t call componentWillReceiveProps() with initial props during mounting. It only calls this method if some of component’s props may update. Calling this.setState() generally doesn’t trigger componentWillReceiveProps().

shouldComponentUpdate()

shouldComponentUpdate(nextProps, nextState)

Use shouldComponentUpdate() to let React know if a component’s output is not affected by the current change in state or props. The default behavior is to re-render on every state change, and in the vast majority of cases you should rely on the default behavior.

shouldComponentUpdate() is invoked before rendering when new props or state are being received. Defaults to true. This method is not called for the initial render or when forceUpdate() is used.

Returning false does not prevent child components from re-rendering when their state changes.

Currently, if shouldComponentUpdate() returns false, then componentWillUpdate(), render(), and componentDidUpdate() will not be invoked. Note that in the future React may treat shouldComponentUpdate() as a hint rather than a strict directive, and returning false may still result in a re-rendering of the component.

If you determine a specific component is slow after profiling, you may change it to inherit from React.PureComponent which implements shouldComponentUpdate() with a shallow prop and state comparison. If you are confident you want to write it by hand, you may compare this.props with nextProps and this.state with nextState and return false to tell React the update can be skipped.

We do not recommend doing deep equality checks or using JSON.stringify() in shouldComponentUpdate(). It is very inefficient and will harm performance.

componentWillUpdate()

componentWillUpdate(nextProps, nextState)

componentWillUpdate() is invoked immediately before rendering when new props or state are being received. Use this as an opportunity to perform preparation before an update occurs. This method is not called for the initial render.

Note that you cannot call this.setState() here; nor should you do anything else (e.g. dispatch a Redux action) that would trigger an update to a React component before componentWillUpdate() returns.

If you need to update state in response to props changes, use componentWillReceiveProps() instead.

Note

componentWillUpdate() will not be invoked if shouldComponentUpdate() returns false.

componentDidUpdate()

componentDidUpdate(prevProps, prevState)

componentDidUpdate() is invoked immediately after updating occurs. This method is not called for the initial render.

Use this as an opportunity to operate on the DOM when the component has been updated. This is also a good place to do network requests as long as you compare the current props to previous props (e.g. a network request may not be necessary if the props have not changed).

Note

componentDidUpdate() will not be invoked if shouldComponentUpdate() returns false.

componentWillUnmount()

componentWillUnmount()

componentWillUnmount() is invoked immediately before a component is unmounted and destroyed. Perform any necessary cleanup in this method, such as invalidating timers, canceling network requests, or cleaning up any subscriptions that were created in componentDidMount().

componentDidCatch()

componentDidCatch(error, info)

Error boundaries are React components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed. Error boundaries catch errors during rendering, in lifecycle methods, and in constructors of the whole tree below them.

A class component becomes an error boundary if it defines this lifecycle method. Calling setState() in it lets you capture an unhandled JavaScript error in the below tree and display a fallback UI. Only use error boundaries for recovering from unexpected exceptions; don’t try to use them for control flow.

For more details, see Error Handling in React 16.

Note

Error boundaries only catch errors in the components below them in the tree. An error boundary can’t catch an error within itself.

setState()

setState(updater[, callback])

setState() enqueues changes to the component state and tells React that this component and its children need to be re-rendered with the updated state. This is the primary method you use to update the user interface in response to event handlers and server responses.

Think of setState() as a request rather than an immediate command to update the component. For better perceived performance, React may delay it, and then update several components in a single pass. React does not guarantee that the state changes are applied immediately.

setState() does not always immediately update the component. It may batch or defer the update until later. This makes reading this.state right after calling setState() a potential pitfall. Instead, use componentDidUpdate or a setState callback (setState(updater, callback)), either of which are guaranteed to fire after the update has been applied. If you need to set the state based on the previous state, read about the updater argument below.

setState() will always lead to a re-render unless shouldComponentUpdate() returns false. If mutable objects are being used and conditional rendering logic cannot be implemented in shouldComponentUpdate(), calling setState() only when the new state differs from the previous state will avoid unnecessary re-renders.

The first argument is an updater function with the signature:

(prevState, props) => stateChange

prevState is a reference to the previous state. It should not be directly mutated. Instead, changes should be represented by building a new object based on the input from prevState and props. For instance, suppose we wanted to increment a value in state by props.step:

this.setState((prevState, props) => {
  return {counter: prevState.counter + props.step};
});

Both prevState and props received by the updater function are guaranteed to be up-to-date. The output of the updater is shallowly merged with prevState.

The second parameter to setState() is an optional callback function that will be executed once setState is completed and the component is re-rendered. Generally we recommend using componentDidUpdate() for such logic instead.

You may optionally pass an object as the first argument to setState() instead of a function:

setState(stateChange[, callback])

This performs a shallow merge of stateChange into the new state, e.g., to adjust a shopping cart item quantity:

this.setState({quantity: 2})

This form of setState() is also asynchronous, and multiple calls during the same cycle may be batched together. For example, if you attempt to increment an item quantity more than once in the same cycle, that will result in the equivalent of:

Object.assign(
  previousState,
  {quantity: state.quantity + 1},
  {quantity: state.quantity + 1},
  ...
)

Subsequent calls will override values from previous calls in the same cycle, so the quantity will only be incremented once. If the next state depends on the previous state, we recommend using the updater function form, instead:

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

For more detail, see the State and Lifecycle guide.

forceUpdate()

component.forceUpdate(callback)

By default, when your component’s state or props change, your component will re-render. If your render() method depends on some other data, you can tell React that the component needs re-rendering by calling forceUpdate().

Calling forceUpdate() will cause render() to be called on the component, skipping shouldComponentUpdate(). This will trigger the normal lifecycle methods for child components, including the shouldComponentUpdate() method of each child. React will still only update the DOM if the markup changes.

Normally you should try to avoid all uses of forceUpdate() and only read from this.props and this.state in render().

Class Properties

defaultProps

defaultProps can be defined as a property on the component class itself, to set the default props for the class. This is used for undefined props, but not for null props. For example:

class CustomButton extends React.Component {
  // ...
}

CustomButton.defaultProps = {
  color: 'blue'
};

If props.color is not provided, it will be set by default to 'blue':

  render() {
    return <CustomButton /> ; // props.color will be set to blue
  }

If props.color is set to null, it will remain null:

  render() {
    return <CustomButton color={null} /> ; // props.color will remain null
  }

displayName

The displayName string is used in debugging messages. Usually, you don’t need to set it explicitly because it’s inferred from the name of the function or class that defines the component. You might want to set it explicitly if you want to display a different name for debugging purposes or when you create a higher-order component, see Wrap the Display Name for Easy Debugging for details.

Instance Properties

props

this.props contains the props that were defined by the caller of this component. See Components and Props for an introduction to props.

In particular, this.props.children is a special prop, typically defined by the child tags in the JSX expression rather than in the tag itself.

state

The state contains data specific to this component that may change over time. The state is user-defined, and it should be a plain JavaScript object.

If you don’t use it in render(), it shouldn’t be in the state. For example, you can put timer IDs directly on the instance.

See State and Lifecycle for more information about the state.

Never mutate this.state directly, as calling setState() afterwards may replace the mutation you made. Treat this.state as if it were immutable.