freeCodeCamp/curriculum/challenges/english/03-front-end-libraries/react/pass-state-as-props-to-child-components.english.md

id	title	challengeType	isRequired
5a24c314108439a4d403617a	Pass State as Props to Child Components	6	false

Description

You saw a lot of examples that passed props to child JSX elements and child React components in previous challenges. You may be wondering where those props come from. A common pattern is to have a stateful component containing the state important to your app, that then renders child components. You want these components to have access to some pieces of that state, which are passed in as props. For example, maybe you have an App component that renders a Navbar, among other components. In your App, you have state that contains a lot of user information, but the Navbar only needs access to the user's username so it can display it. You pass that piece of state to the Navbar component as a prop. This pattern illustrates some important paradigms in React. The first is unidirectional data flow. State flows in one direction down the tree of your application's components, from the stateful parent component to child components. The child components only receive the state data they need. The second is that complex stateful apps can be broken down into just a few, or maybe a single, stateful component. The rest of your components simply receive state from the parent as props, and render a UI from that state. It begins to create a separation where state management is handled in one part of code and UI rendering in another. This principle of separating state logic from UI logic is one of React's key principles. When it's used correctly, it makes the design of complex, stateful applications much easier to manage.

Instructions

The MyApp component is stateful and renders a Navbar component as a child. Pass the name property in its state down to the child component, then show the name in the h1 tag that's part of the Navbar render method.

Tests

tests:
  - text: The <code>MyApp</code> component should render with a <code>Navbar</code> component inside.
    testString: assert((function() { const mockedComponent = Enzyme.mount(React.createElement(MyApp)); return mockedComponent.find('MyApp').length === 1 && mockedComponent.find('Navbar').length === 1; })(), 'The <code>MyApp</code> component should render with a <code>Navbar</code> component inside.');
  - text: The <code>Navbar</code> component should receive the <code>MyApp</code> state property <code>name</code> as props.
    testString: 'async () => { const waitForIt = (fn) => new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250)); const mockedComponent = Enzyme.mount(React.createElement(MyApp)); const setState = () => { mockedComponent.setState({name: ''TestName''}); return waitForIt(() => mockedComponent.find(''Navbar'').props() )}; const navProps = await setState(); assert(navProps.name === ''TestName'', ''The <code>Navbar</code> component should receive the <code>MyApp</code> state property <code>name</code> as props.''); }; '
  - text: The <code>h1</code> element in <code>Navbar</code> should render the <code>name</code> prop.
    testString: 'async () => { const waitForIt = (fn) => new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 250)); const mockedComponent = Enzyme.mount(React.createElement(MyApp)); const navH1Before = mockedComponent.find(''Navbar'').find(''h1'').text(); const setState = () => { mockedComponent.setState({name: ''TestName''}); return waitForIt(() => mockedComponent.find(''Navbar'').find(''h1'').text() )}; const navH1After = await setState(); assert(new RegExp(''TestName'').test(navH1After) && navH1After !== navH1Before, ''The <code>h1</code> element in <code>Navbar</code> should render the <code>name</code> prop.''); }; '

Challenge Seed

class MyApp extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      name: 'CamperBot'
    }
  }
  render() {
    return (
       <div>
         <Navbar /* your code here */ />
       </div>
    );
  }
};

class Navbar extends React.Component {
  constructor(props) {
    super(props);
  }
  render() {
    return (
    <div>
      <h1>Hello, my name is: /* your code here */ </h1>
    </div>
    );
  }
};

After Test

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

Solution

class MyApp extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      name: 'CamperBot'
    }
  }
  render() {
    return (
       <div>
         <Navbar name={this.state.name}/>
       </div>
    );
  }
};
class Navbar extends React.Component {
  constructor(props) {
    super(props);
  }
  render() {
    return (
    <div>
      <h1>Hello, my name is: {this.props.name}</h1>
    </div>
    );
  }
};