What are Composition and Inheritence?

• Inheritence comes from a class's ancestors
• Composition is data passed down to children as props

Composition vs Inheritance

• Inheritance can be overused
• Composition of behavior can be simpler and easier
• React supports using composition over deep inheritance

Composition - Using props.children

• Any Components within <FancyBorder> become props.children
• The parent component can wrap children in a <div> for layout/style

Composing Behavior with Components

/* FancyBorder will compose with children*/
function FancyBorder(props) {
  return (
    <div className={'FancyBorder FancyBorder-' + props.color}>
      {props.children}
    </div>
  );
}

/* WelcomeDialog uses FancyBorder with children*/
function WelcomeDialog() {
  return (
    <FancyBorder color="blue">
      <h1 className="Dialog-title">
        Welcome
      </h1>
      <p className="Dialog-message">
        Thank you for visiting our spacecraft!
      </p>
    </FancyBorder>
  );
}

Composition - Child Groups

• Sometimes components have children in several places
• Assigning the child components to prop names can help organize

function SplitPane(props) {
  return (
    <div className="SplitPane">
      <div className="SplitPane-left">
        {props.left}
      </div>
      <div className="SplitPane-right">
        {props.right}
      </div>
    </div>
  );
}

function App() {
  return (
    <SplitPane
      left={
        <Contacts />
      }
      right={
        <Chat />
      } />
  );
}

Composition - Classes Work Too

• Both Stateless Functions and Classes compose
• Specific Components can configure General Components
• Props can be used to configure

Composition - When Inheritance?

• Facebook doesn't use inheritance beyond initial Component Classes
• Frontends can be built using a mixture of 'general' and 'specific' components
• Use Props to pass specific attributes and children to render specific behavior
• Behavior shared within Components such as logic or utilities can be shared as JavaScript libraries or modules
• Using import is more useful and less restricting then extends