Uncaught ReferenceError: ReactDOM is not defined

It may be the spelling issue - it is ReactDOM, not ReactDom.

This has changed over time with the new release of React

ReactDOM available since version 0.14.0, so you need to use React.render (because you have a React version 0.13.3) instead,

setInterval(function() {
  React.render(
    <HelloWorld date={new Date()} />,
    document.getElementById('example')
  );
}, 500);

or upgrade your React version and include ReactDOM

Changes in React 0.14

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