Sudenkuoppa

We recommend defining components as functions instead of classes. See how to migrate.

PureComponent is similar to 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.

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

Viite

PureComponent

To skip re-rendering a class component for same props and state, extend PureComponent instead of Component:

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. Extending PureComponent is equivalent to defining a custom shouldComponentUpdate method that shallowly compares props and state.

See more examples below.

Käyttö

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 can opt into this behavior by extending PureComponent:

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

A React component should always have pure rendering logic. 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):

import { PureComponent, useState } from 'react';

class Greeting extends PureComponent {
  render() {
    console.log("Greeting was rendered at", new Date().toLocaleTimeString());
    return <h3>Hello{this.props.name && ', '}{this.props.name}!</h3>;
  }
}

export default function MyApp() {
  const [name, setName] = useState('');
  const [address, setAddress] = useState('');
  return (
    <>
      <label>
        Name{': '}
        <input value={name} onChange={e => setName(e.target.value)} />
      </label>
      <label>
        Address{': '}
        <input value={address} onChange={e => setAddress(e.target.value)} />
      </label>
      <Greeting name={name} />
    </>
  );
}

Sudenkuoppa

We recommend defining components as functions instead of classes. See how to migrate.

Alternatives

Migrating from a PureComponent class component to a function

We recommend using function components instead of class components in new code. If you have some existing class components using PureComponent, here is how you can convert them. This is the original code:

import { PureComponent, useState } from 'react';

class Greeting extends PureComponent {
  render() {
    console.log("Greeting was rendered at", new Date().toLocaleTimeString());
    return <h3>Hello{this.props.name && ', '}{this.props.name}!</h3>;
  }
}

export default function MyApp() {
  const [name, setName] = useState('');
  const [address, setAddress] = useState('');
  return (
    <>
      <label>
        Name{': '}
        <input value={name} onChange={e => setName(e.target.value)} />
      </label>
      <label>
        Address{': '}
        <input value={address} onChange={e => setAddress(e.target.value)} />
      </label>
      <Greeting name={name} />
    </>
  );
}

When you convert this component from a class to a function, wrap it in memo:

import { memo, useState } from 'react';

const Greeting = memo(function Greeting({ name }) {
  console.log("Greeting was rendered at", new Date().toLocaleTimeString());
  return <h3>Hello{name && ', '}{name}!</h3>;
});

export default function MyApp() {
  const [name, setName] = useState('');
  const [address, setAddress] = useState('');
  return (
    <>
      <label>
        Name{': '}
        <input value={name} onChange={e => setName(e.target.value)} />
      </label>
      <label>
        Address{': '}
        <input value={address} onChange={e => setAddress(e.target.value)} />
      </label>
      <Greeting name={name} />
    </>
  );
}

Huomaa

Unlike PureComponent, memo does not compare the new and the old state. In function components, calling the set function with the same state already prevents re-renders by default, even without memo.

PreviousisValidElement

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