freeCodeCamp

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id	title	challengeType	isRequired
5a24c314108439a4d403617d	Use the Lifecycle Method componentDidMount	6	false

Description

Most web developers, at some point, need to call an API endpoint to retrieve data. If you're working with React, it's important to know where to perform this action. The best practice with React is to place API calls or any calls to your server in the lifecycle method componentDidMount(). This method is called after a component is mounted to the DOM. Any calls to setState() here will trigger a re-rendering of your component. When you call an API in this method, and set your state with the data that the API returns, it will automatically trigger an update once you receive the data.

Instructions

There is a mock API call in componentDidMount(). It sets state after 2.5 seconds to simulate calling a server to retrieve data. This example requests the current total active users for a site. In the render method, render the value of activeUsers in the h1. Watch what happens in the preview, and feel free to change the timeout to see the different effects.

Tests

tests:
  - text: <code>MyComponent</code> should render a <code>div</code> element which wraps an <code>h1</code> tag.
    testString: assert((() => { const mockedComponent = Enzyme.mount(React.createElement(MyComponent)); return (mockedComponent.find('div').length === 1 && mockedComponent.find('h1').length === 1); })(), '<code>MyComponent</code> should render a <code>div</code> element which wraps an <code>h1</code> tag.');
  - text: Component state should be updated with a timeout function in <code>componentDidMount</code>.
    testString: assert((() => { const mockedComponent = Enzyme.mount(React.createElement(MyComponent)); return new RegExp('setTimeout(.|\n)+setState(.|\n)+activeUsers').test(String(mockedComponent.instance().componentDidMount)); })(), 'Component state should be updated with a timeout function in <code>componentDidMount</code>.');
  - text: The <code>h1</code> tag should render the <code>activeUsers</code> value from <code>MyComponent</code>&apos;s state.
    testString: 'async () => { const mockedComponent = Enzyme.mount(React.createElement(MyComponent)); const first = () => { mockedComponent.setState({ activeUsers: 1237 }); return mockedComponent.find(''h1'').text(); }; const second = () => { mockedComponent.setState({ activeUsers: 1000 }); return mockedComponent.find(''h1'').text(); }; assert(new RegExp(''1237'').test(first()) && new RegExp(''1000'').test(second()), ''The <code>h1</code> tag should render the <code>activeUsers</code> value from <code>MyComponent</code>&apos;s state.''); }; '

Challenge Seed

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      activeUsers: null
    };
  }
  componentDidMount() {
    setTimeout( () => {
      this.setState({
        activeUsers: 1273
      });
    }, 2500);
  }
  render() {
    return (
      <div>
        <h1>Active Users: { /* change code here */ }</h1>
      </div>
    );
  }
};

After Test

ReactDOM.render(<MyComponent />, document.getElementById('root'))

Solution

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      activeUsers: null
    };
  }
  componentDidMount() {
    setTimeout( () => {
      this.setState({
        activeUsers: 1273
      });
    }, 2500);
  }
  render() {
    return (
      <div>
        <h1>Active Users: {this.state.activeUsers}</h1>
      </div>
    );
  }
};

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