React Components

• Isolated pieces of a website or app
• Can be passed data from parent components in the form of props
• Can be rendered manually, or programatically
• Return JSX

function Profile (props) {
  const [userData, setUserData] = useState(data) 

  return(
    <Header>
      <Profile
        name={userData.name}
        interests={userData.interests}
        description={userData.description}
      />
      <Activity />
      <Contact />
    </Header>
  )
}

Using Components

• Typically the index.js initiates the components tree from the top level
• Can be made up of other components
• Child components are rendered by their parents
• Parents pass stateful data as props to children

Using Arrays of Components

• Many components can be rendered at once
• Wrap the components in an array
• React will iterate over and render each

function ProfileList (props) {
  return(
    <Header>
      [
        <Profile userId="One"/>,
        <Profile userId="Two"/>,
        <Profile userId="Three"/>
      ]
    </Header>
  )
}

Building components with Iteration

• Iteration and Loops can be used just like in plain JavaScript
• Many components can be built based on collections of data
• Components can be conditionally rendered

function Comment (props) {
  return (
    <li>
      <p>User: {props.user}</p>
      <p>Comment: {props.content}</p>
    </li>
  ) 
}

function CommentList (props) {

  const comments = [
    { user: 'Joshua', content: 'Components are my fave!' },
    { user: 'Ada', content: 'Yes they make life easy' },
    { user: 'Alex', content: 'Loved them for years' }
  ];

  const commentComponents = comments.map((comment, index) => {
    return <Comment user={ comment.user } content={ comment.content }/>
  });

  return (
    <div className="comments">
      <h2>Comment List</h2>
      <ul>
        { commentComponents }
      </ul>
    </div>
  )
}

Immutable Components

• Once components are rendered they cannot be updated
• Re-rendering the component is how to update them
• Re-renders are triggered when state or props change

function Tick(props) {
  [date, setDate] = useState(new Date().toLocaleTimeString())

  useEffect(() => {
    setTimeout(() => {
      setDate(new Date().toLocaleTimeString())
    }, 1000)
  })

  return (
    <div>
      <h1>Hello, world!</h1>
      <h2>It is {date}.</h2>
    </div>
  )
}

Ticking Clock Live

See the Pen React Clock Example by Joshua Burke (@Dangeranger) on CodePen.

https://codepen.io/Dangeranger/pen/gjdNEB

Granular Render Updates

Component Lifecycle

Mounting

• constructor()
• static getDerivedStateFromProps()
• render()
• componentDidMount()

Updating

• static getDerivedStateFromProps()
• shouldComponentUpdate()
• render()
• getSnapshotBeforeUpdate()
• componentDidUpdate()

Unmounting

• componentWillUnmount()

Errors

• componentDidCatch()

Lifecycle Methods Diagram

http://projects.wojtekmaj.pl/react-lifecycle-methods-diagram/