<!DOCTYPE html>
<html>

  <head>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.0.1/react.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.0.1/react-dom.js"></script>
  </head>

  <body>
    <div id="container">
      <HelloComponent />
    </div>
    <script src="hello-component.js"></script>
  </body>

</html>
function HelloComponent() { return <div>Hello there</div> } 

ReactDOM.render(
  <HelloComponent />,
  document.getElementById("container")
);