Call external Javascript function from react components

In index.html

<script type="text/javascript">
  function test(){
    alert('Function from index.html');
  }
</script>

In your component

componentWillMount() {
  window.test();
}

2022 Typescript edit:

Create a file global.d.ts like so (doc):

interface Window {
  test: () => void;
}

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