start js function when clicked html code example

Example 1: how to call a function with a button in javascript

<script>
		function myFunction()
		{
			document.write("<h1>HELLO</h1>")
		}
	</script>
<button onclick="myFunction()">Click</button>

Example 2: javascript onclick button

var button = document.querySelector('button');
button.onclick = function() {
  //do stuff
}