This section is incomplete, please see the old docs for [React.PureComponent.](https://reactjs.org/docs/react-api.html#reactpurecomponent)
</Wip>
We recommend to define components as functions instead of classes. [See how to migrate.](#alternatives)
</Pitfall>
<Intro>
React.PureComponent is similar to React.Component. The difference between them is that React.Component doesn’t implement shouldComponentUpdate(), but React.PureComponent implements it with a shallow prop and state comparison.
If your React component’s render() function renders the same result given the same props and state, you can use React.PureComponent for a performance boost in some cases.
`PureComponent` is similar to [`Component`](/apis/react/Component) but it skips re-renders for same props and state. Class components are still supported by React, but we don't recommend using them in new code.
```js
class Welcome extends React.PureComponent {
class Greeting extends PureComponent {
render() {
return <h1>Hello, {this.props.name}</h1>;
return <h1>Hello, {this.props.name}!</h1>;
}
}
```
@ -26,3 +23,188 @@ class Welcome extends React.PureComponent {
</Intro>
<InlineToc/>
---
## Usage {/*usage*/}
### Skipping unnecessary re-renders for class components {/*skipping-unnecessary-re-renders-for-class-components*/}
React normally re-renders a component whenever its parent re-renders. As an optimization, you can create a component that React will not re-render when its parent re-renders so long as its new props and state are the same as the old props and state. [Class components](/apis/react/Component) can opt into this behavior by extending `PureComponent`:
```js {1}
class Greeting extends PureComponent {
render() {
return <h1>Hello, {this.props.name}!</h1>;
}
}
```
A React component should always have [pure rendering logic.](/learn/keeping-components-pure) This means that it must return the same output if its props, state, and context haven't changed. By using `PureComponent`, you are telling React that your component complies with this requirement, so React doesn't need to re-render as long as its props and state haven't changed. However, your component will still re-render if a context that it's using changes.
In this example, notice that the `Greeting` component re-renders whenever `name` is changed (because that's one of its props), but not when `address` is changed (because it's not passed to `Greeting` as a prop):
<Sandpack>
```js
import { PureComponent, useState } from 'react';
class Greeting extends PureComponent {
render() {
console.log("Greeting was rendered at", new Date().toLocaleTimeString());
We recommend to define components as functions instead of classes. [See how to migrate.](#alternatives)
</Pitfall>
---
## Alternatives {/*alternatives*/}
### Migrating from a `PureComponent` class component to a function {/*migrating-from-a-purecomponent-class-component-to-a-function*/}
We recommend to use function components instead of [class components](/apis/react/Component) in the new code. If you have some existing class components using `PureComponent`, here is how you can convert them. This is the original code:
<Sandpack>
```js
import { PureComponent, useState } from 'react';
class Greeting extends PureComponent {
render() {
console.log("Greeting was rendered at", new Date().toLocaleTimeString());
Unlike `PureComponent`, [`memo`](/apis/react/memo) does not compare the new and the old state. In function components, calling the [`set` function](/apis/react/useState#setstate) with the same state [already prevents re-renders by default,](/apis/react/memo#updating-a-memoized-component-using-state) even without `memo`.
</Note>
---
## Reference {/*reference*/}
### `PureComponent` {/*purecomponent*/}
To skip re-rendering a class component for same props and state, extend `PureComponent` instead of [`Component`:](/apis/react/Component)
```js
import { PureComponent } from 'react';
class Greeting extends PureComponent {
render() {
return <h1>Hello, {this.props.name}!</h1>;
}
}
```
`PureComponent` is a subclass of `Component` and supports [all the `Component` APIs.](/apis/react/Component#reference) Extending `PureComponent` is equivalent to defining a custom [`shouldComponentUpdate`](/apis/react/Component#shouldcomponentupdate) method that shallowly compares props and state.
dan
2 years ago
GitHub