freeCodeCamp/curriculum/challenges/chinese/03-front-end-libraries/react/optimize-re-renders-with-shouldcomponentupdate.md

---
id: 5a24c314108439a4d4036180
title: Optimize Re-Renders with shouldComponentUpdate
challengeType: 6
forumTopicId: 301398
dashedName: optimize-re-renders-with-shouldcomponentupdate
---

# --description--

So far, if any component receives new `state` or new `props`, it re-renders itself and all its children. This is usually okay. But React provides a lifecycle method you can call when child components receive new `state` or `props`, and declare specifically if the components should update or not. The method is `shouldComponentUpdate()`, and it takes `nextProps` and `nextState` as parameters.

This method is a useful way to optimize performance. For example, the default behavior is that your component re-renders when it receives new `props`, even if the `props` haven't changed. You can use `shouldComponentUpdate()` to prevent this by comparing the `props`. The method must return a `boolean` value that tells React whether or not to update the component. You can compare the current props (`this.props`) to the next props (`nextProps`) to determine if you need to update or not, and return `true` or `false` accordingly.

# --instructions--

The `shouldComponentUpdate()` method is added in a component called `OnlyEvens`. Currently, this method returns `true` so `OnlyEvens` re-renders every time it receives new `props`. Modify the method so `OnlyEvens` updates only if the `value` of its new props is even. Click the `Add` button and watch the order of events in your browser's console as the lifecycle hooks are triggered.

# --hints--

The `Controller` component should render the `OnlyEvens` component as a child.

```js
assert(
  (() => {
    const mockedComponent = Enzyme.mount(React.createElement(Controller));
    return (
      mockedComponent.find('Controller').length === 1 &&
      mockedComponent.find('OnlyEvens').length === 1
    );
  })()
);
```

The `shouldComponentUpdate` method should be defined on the `OnlyEvens` component.

```js
assert(
  (() => {
    const child = React.createElement(OnlyEvens)
      .type.prototype.shouldComponentUpdate.toString()
      .replace(/s/g, '');
    return child !== 'undefined';
  })()
);
```

The `OnlyEvens` component should return an `h1` tag which renders the value of `this.props.value`.

```js
(() => {
  const mockedComponent = Enzyme.mount(React.createElement(Controller));
  const first = () => {
    mockedComponent.setState({ value: 1000 });
    return mockedComponent.find('h1').html();
  };
  const second = () => {
    mockedComponent.setState({ value: 10 });
    return mockedComponent.find('h1').html();
  };
  const firstValue = first();
  const secondValue = second();
  assert(firstValue === '<h1>1000</h1>' && secondValue === '<h1>10</h1>');
})();
```

`OnlyEvens` should re-render only when `nextProps.value` is even.

```js
(() => {
  const mockedComponent = Enzyme.mount(React.createElement(Controller));
  const first = () => {
    mockedComponent.setState({ value: 8 });
    return mockedComponent.find('h1').text();
  };
  const second = () => {
    mockedComponent.setState({ value: 7 });
    return mockedComponent.find('h1').text();
  };
  const third = () => {
    mockedComponent.setState({ value: 42 });
    return mockedComponent.find('h1').text();
  };
  const firstValue = first();
  const secondValue = second();
  const thirdValue = third();
  assert(firstValue === '8' && secondValue === '8' && thirdValue === '42');
})();
```

# --seed--

## --after-user-code--

```jsx
ReactDOM.render(<Controller />, document.getElementById('root'));
```

## --seed-contents--

```jsx
class OnlyEvens extends React.Component {
  constructor(props) {
    super(props);
  }
  shouldComponentUpdate(nextProps, nextState) {
    console.log('Should I update?');
    // Change code below this line
    return true;
    // Change code above this line
  }
  componentDidUpdate() {
    console.log('Component re-rendered.');
  }
  render() {
    return <h1>{this.props.value}</h1>;
  }
}

class Controller extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      value: 0
    };
    this.addValue = this.addValue.bind(this);
  }
  addValue() {
    this.setState(state => ({
      value: state.value + 1
    }));
  }
  render() {
    return (
      <div>
        <button onClick={this.addValue}>Add</button>
        <OnlyEvens value={this.state.value} />
      </div>
    );
  }
}
```

# --solutions--

```jsx
class OnlyEvens extends React.Component {
  constructor(props) {
    super(props);
  }
  shouldComponentUpdate(nextProps, nextState) {
    console.log('Should I update?');
    // Change code below this line
    return nextProps.value % 2 === 0;
    // Change code above this line
  }
  componentDidUpdate() {
    console.log('Component re-rendered.');
  }
  render() {
    return <h1>{this.props.value}</h1>;
  }
}

class Controller extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      value: 0
    };
    this.addValue = this.addValue.bind(this);
  }
  addValue() {
    this.setState(state => ({
      value: state.value + 1
    }));
  }
  render() {
    return (
      <div>
        <button onClick={this.addValue}>Add</button>
        <OnlyEvens value={this.state.value} />
      </div>
    );
  }
}
```