react.dev/docs/ref-03-component-specs.md

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Component Specifications

When creating a component class by invoking React.createClass(), you should provide a specification object that contains a render method and can optionally contain other lifecycle methods described here.

render

ReactComponent render()

The render() method is required.

When called, it should examine this.props and this.state and return a single child component. This child component can be either a native DOM component (such as <div>) or another composite component that you've defined yourself.

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 read from or write to the DOM or otherwise interact with the browser (e.g., by using setTimeout). If you need to interact with the browser, perform your work in componentDidMount() or the other lifecycle methods instead. Keeping render() pure makes server rendering more practical and makes components easier to think about.

getInitialState

object getInitialState()

Invoked once when the component is mounted. The return value will be used as the initial value of this.state.

getDefaultProps

object getDefaultProps()

Invoked once when the component is mounted. Values in the mapping will be set on this.props if that prop is not specified by the parent component (i.e. using an in check).

This method is invoked before getInitialState and therefore cannot rely on this.state or use this.setState.

propTypes

object propTypes

The propTypes object allows you to validate props being passed to your components. For more information about propTypes, see Reusable Components.

mixins

array mixins

The mixins array allows you to use mixins to share behavior among multiple components. For more information about mixins, see Reusable Components.

Lifecycle Methods

Various methods are executed at specific points in a component's lifecycle.

Mounting: componentWillMount

componentWillMount()

Invoked immediately before rendering occurs. If you call setState within this method, render() will see the updated state and will be executed only once despite the state change.

Mounting: componentDidMount

componentDidMount(DOMElement rootNode)

Invoked immediately after rendering occurs. At this point in the lifecycle, the component has a DOM representation which you can access via the rootNode argument or by calling this.getDOMNode().

If you want to integrate with other JavaScript frameworks, set timers using setTimeout or setInterval, or send AJAX requests, perform those operations in this method.

Updating: componentWillReceiveProps

componentWillReceiveProps(object nextProps)

Invoked when a component is receiving new props. This method is not called for the initial render.

Use this as an opportunity to react to a prop transition before render() is called by updating the state using this.setState(). The old props can be accessed via this.props. Calling this.setState() within this function will not trigger an additional render.

componentWillReceiveProps: function(nextProps) {
  this.setState({
    likesIncreasing: nextProps.likeCount > this.props.likeCount
  });
}

Note:

There is no analogous method componentWillReceiveState. An incoming prop transition may cause a state change, but the opposite is not true. If you need to perform operations in response to a state change, use componentWillUpdate.

Updating: shouldComponentUpdate

boolean shouldComponentUpdate(object nextProps, object nextState)

Invoked before rendering when new props or state are being received. This method is not called for the initial render or when forceUpdate is used.

Use this as an opportunity to return false when you're certain that the transition to the new props and state will not require a component update.

shouldComponentUpdate: function(nextProps, nextState) {
  return !equal(nextProps, this.props) || !equal(nextState, this.state);
}

If shouldComponentUpdate returns false, then render() will be completely skipped until the next state change. (In addition, componentWillUpdate and componentDidUpdate will not be called.)

By default, shouldComponentUpdate always returns true to prevent subtle bugs when state is mutated in place, but if you are careful to always treat state as immutable and to read only from props and state in render() then you can override shouldComponentUpdate with an implementation that compares the old props and state to their replacements.

If performance is a bottleneck, especially with dozens or hundreds of components, use shouldComponentUpdate to speed up your app.

Updating: componentWillUpdate

componentWillUpdate(object nextProps, object nextState)

Invoked immediately before rendering when new props or state are being received. This method is not called for the initial render.

Use this as an opportunity to perform preparation before an update occurs.

Note:

You cannot use this.setState() in this method. If you need to update state in response to a prop change, use componentWillReceiveProps instead.

Updating: componentDidUpdate

componentDidUpdate(object prevProps, object prevState, DOMElement rootNode)

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.

Unmounting: componentWillUnmount

componentWillUnmount()

Invoked immediately before a component is unmounted from the DOM.

Perform any necessary cleanup in this method, such as invalidating timers or cleaning up any DOM elements that were created in componentDidMount.