---
id: 5a24c314108439a4d4036180
title: Optimize Re-Renders with shouldComponentUpdate
challengeType: 6
isRequired: false
---

## Description
<section id='description'>
So far, if any component receives new <code>state</code> or new <code>props</code>, 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 <code>state</code> or <code>props</code>, and declare specifically if the components should update or not. The method is <code>shouldComponentUpdate()</code>, and it takes <code>nextProps</code> and <code>nextState</code> 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 <code>props</code>, even if the <code>props</code> haven't changed. You can use <code>shouldComponentUpdate()</code> to prevent this by comparing the <code>props</code>. The method must return a <code>boolean</code> value that tells React whether or not to update the component. You can compare the current props (<code>this.props</code>) to the next props (<code>nextProps</code>) to determine if you need to update or not, and return <code>true</code> or <code>false</code> accordingly.
</section>

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

## Tests
<section id='tests'>

```yml
tests:
  - text: The <code>Controller</code> component should render the <code>OnlyEvens</code> component as a child.
    testString: assert((() => { const mockedComponent = Enzyme.mount(React.createElement(Controller)); return mockedComponent.find('Controller').length === 1 && mockedComponent.find('OnlyEvens').length === 1; })(), 'The <code>Controller</code> component should render the <code>OnlyEvens</code> component as a child.');
  - text: The <code>shouldComponentUpdate</code> method should be defined on the <code>OnlyEvens</code> component.
    testString: assert((() => { const child = React.createElement(OnlyEvens).type.prototype.shouldComponentUpdate.toString().replace(/s/g,''); return child !== 'undefined'; })(), 'The <code>shouldComponentUpdate</code> method should be defined on the <code>OnlyEvens</code> component.');
  - text: The <code>OnlyEvens</code> component should return an <code>h1</code> tag which renders the value of <code>this.props.value</code>.
    testString: 'async () => { const waitForIt = (fn) => new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250)); const mockedComponent = Enzyme.mount(React.createElement(Controller)); const first = () => { mockedComponent.setState({ value: 1000 }); return waitForIt(() => mockedComponent.find(''h1'').html()); }; const second = () => { mockedComponent.setState({ value: 10 }); return waitForIt(() => mockedComponent.find(''h1'').html()); }; const firstValue = await first(); const secondValue = await second(); assert(firstValue === ''<h1>1000</h1>'' && secondValue === ''<h1>10</h1>'', ''The <code>OnlyEvens</code> component should return an <code>h1</code> tag which renders the value of <code>this.props.value</code>.''); }; '
  - text: <code>OnlyEvens</code> should re-render only when <code>nextProps.value</code> is even.
    testString: 'async () => { const waitForIt = (fn) => new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250)); const mockedComponent = Enzyme.mount(React.createElement(Controller)); const first = () => { mockedComponent.setState({ value: 8 }); return waitForIt(() => mockedComponent.find(''h1'').text()); }; const second = () => { mockedComponent.setState({ value: 7 }); return waitForIt(() => mockedComponent.find(''h1'').text()); }; const third = () => { mockedComponent.setState({ value: 42 }); return waitForIt(() => mockedComponent.find(''h1'').text()); }; const firstValue = await first(); const secondValue = await second(); const thirdValue = await third(); assert(firstValue === ''8'' && secondValue === ''8'' && thirdValue === ''42'', ''<code>OnlyEvens</code> should re-render only when <code>nextProps.value</code> is even.''); }; '

```

</section>

## Challenge Seed
<section id='challengeSeed'>

<div id='jsx-seed'>

```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
  }
  componentWillReceiveProps(nextProps) {
    console.log('Receiving new props...');
  }
  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({
      value: this.state.value + 1
    });
  }
  render() {
    return (
      <div>
        <button onClick={this.addValue}>Add</button>
        <OnlyEvens value={this.state.value}/>
      </div>
    );
  }
};
```

</div>


### After Test
<div id='jsx-teardown'>

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

</div>

</section>

## Solution
<section id='solution'>


```js
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
  }
  componentWillReceiveProps(nextProps) {
    console.log('Receiving new props...');
  }
  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({
      value: this.state.value + 1
    });
  }
  render() {
    return (
      <div>
        <button onClick={this.addValue}>Add</button>
        <OnlyEvens value={this.state.value}/>
      </div>
    );
  }
};
```

</section>