Component Specs and Lifecycle

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.

Note:

It is also possible to use plain JavaScript classes as component classes. These classes can implement most of the same methods, though there are some differences. For more information about these differences, please read our documentation about ES6 classes.

render #

ReactElement render()

The render() method is required.

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

You can also return null or false to indicate that you don't want anything rendered. Behind the scenes, React renders a <noscript> tag to work with our current diffing algorithm. 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 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 before the component is mounted. The return value will be used as the initial value of this.state.

getDefaultProps #

object getDefaultProps()

Invoked once and cached when the class is created. 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 any instances are created and thus cannot rely on this.props. In addition, be aware that any complex objects returned by getDefaultProps() will be shared across instances, not copied.

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.

statics #

object statics

The statics object allows you to define static methods that can be called on the component class. For example:

var MyComponent = React.createClass({
  statics: {
    customMethod: function(foo) {
      return foo === 'bar';
    }
  },
  render: function() {
  }
});

MyComponent.customMethod('bar');  // true

Methods defined within this block are static, meaning that you can run them before any component instances are created, and the methods do not have access to the props or state of your components. If you want to check the value of props in a static method, have the caller pass in the props as an argument to the static method.

displayName #

string displayName

The displayName string is used in debugging messages. JSX sets this value automatically; see JSX in Depth.

Lifecycle Methods #

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

Mounting: componentWillMount #

void componentWillMount()

Invoked once, both on the client and server, immediately before the initial 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 #

void componentDidMount()

Invoked once, only on the client (not on the server), immediately after the initial rendering occurs. At this point in the lifecycle, you can access any refs to your children (e.g., to access the underlying DOM representation). The componentDidMount() method of child components is invoked before that of parent components.

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 #

void 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:

One common mistake is for code executed during this lifecycle method to assume that props have changed. To understand why this is invalid, read A implies B does not imply B implies A

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 nextProps.id !== this.props.id;
}

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 #

void 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 #

void componentDidUpdate(
  object prevProps, object prevState
)

Invoked immediately after the component's updates are flushed to the DOM. 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 #

void 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.