Instances of a React Component are created internally in React when rendering. These instances are reused in subsequent renders, and can be accessed in your component methods as this
. The only way to get a handle to a React Component instance outside of React is by storing the return value of ReactDOM.render
. Inside other Components, you may use refs to achieve the same result.
void setState(
function|object nextState,
[function callback]
)
Performs a shallow merge of nextState into current state. This is the primary method you use to trigger UI updates from event handlers and server request callbacks.
The first argument can be an object (containing zero or more keys to update) or a function (of state and props) that returns an object containing keys to update.
Here is the simple object usage:
setState({mykey: 'my new value'});
It's also possible to pass a function with the signature function(state, props)
. This can be useful in some cases when you want to enqueue an atomic update that consults the previous value of state+props before setting any values. For instance, suppose we wanted to increment a value in state:
setState(function(previousState, currentProps) {
return {myInteger: previousState.myInteger + 1};
});
The second (optional) parameter is a callback function that will be executed once setState
is completed and the component is re-rendered.
Notes:
NEVER mutate
this.state
directly, as callingsetState()
afterwards may replace the mutation you made. Treatthis.state
as if it were immutable.
setState()
does not immediately mutatethis.state
but creates a pending state transition. Accessingthis.state
after calling this method can potentially return the existing value.There is no guarantee of synchronous operation of calls to
setState
and calls may be batched for performance gains.
setState()
will always trigger a re-render unless conditional rendering logic is implemented inshouldComponentUpdate()
. If mutable objects are being used and the logic cannot be implemented inshouldComponentUpdate()
, callingsetState()
only when the new state differs from the previous state will avoid unnecessary re-renders.
void replaceState(
object nextState,
[function callback]
)
Like setState()
but deletes any pre-existing state keys that are not in nextState.
Note:
This method is not available on ES6
class
components that extendReact.Component
. It may be removed entirely in a future version of React.
void forceUpdate(
[function callback]
)
By default, when your component's state or props change, your component will re-render. However, if these change implicitly (eg: data deep within an object changes without changing the object itself) or if your render()
method depends on some other data, you can tell React that it needs to re-run render()
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()
. This makes your component "pure" and your application much simpler and more efficient.
boolean isMounted()
isMounted()
returns true
if the component is rendered into the DOM, false
otherwise. You can use this method to guard asynchronous calls to setState()
or forceUpdate()
.
Note:
This method is not available on ES6
class
components that extendReact.Component
. It will likely be removed entirely in a future version of React, so you might as well start migrating away from isMounted() now.