How to call a JavaScript function in HTML?

To call a JavaScript function in HTML, add onclick="myFunction()" to a button.

First define your function inside a script somewhere above the button like so:

<script>
  function myFunction() {
    alert('Hello World!');
  }
</script>
<button>I am a button!</button>

Then add the onclick listener, to call your function when the button is clicked:

<script>
  function myFunction() {
    alert('Hello World!');
  }
</script>
<button onclick="myFunction()">I am a button!</button>