<div id="root">
    <!-- This element's contents will be replaced with your component. -->
</div>
function Clock(props) {
  return (
    <div>
      <h1>Hello, world!</h1>
      <h2>It is {props.date.toLocaleTimeString()}.</h2>
    </div>
  );
}

function tick() {
  ReactDOM.render(
    <Clock date={new Date()} />,
    document.getElementById('root')
  );
}

setInterval(tick, 1000);
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External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

  1. https://unpkg.com/react/umd/react.development.js
  2. https://unpkg.com/react-dom/umd/react-dom.development.js