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react-from-zero
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06-property-types.html

<!doctype html>

<title>06 Property Types - React From Zero</title>

<script src="https://unpkg.com/react@16.4.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16.4.0/umd/react-dom.development.js"></script>
<script src="https://unpkg.com/@babel/standalone/babel.min.js"></script>

<script src="https://unpkg.com/prop-types@15.6.1/prop-types.js">
// PropTypes were removed from React 16 and are now their own package
</script>

<div id="app"></div>

<script type="text/babel">
// Components get created to encapsulate stuff that should be together in one
// place and for reuse.
// Reuse requires the user of the component to supply the correct properties so
// we can define a type of each property and set defaults

function MyComponent(props) {
  return (
    <div className={props.className}>
      <h1>Hello</h1>
      <h2>{props.customData}</h2>
    </div>
  )
}

// Add the propTypes (function-)property to the component function
// to let it validate its (element-)properties
MyComponent.propTypes = {
  // React supplies us with a bunch of types, like string
  customData: PropTypes.string,
}

// Add defaultProps (function-)property to set the defaults
// if nothing was provided by the user
MyComponent.defaultProps = {
  customData: "default",
  className: "default-class",
}

// This will show a warning in the console, because customData should be a string
var reactElement = <MyComponent customData={123}/>

// This will use the defaults
reactElement = <MyComponent/>

var renderTarget = document.getElementById("app")

ReactDOM.render(reactElement, renderTarget)
</script>