<!DOCTYPE html>
<html>
<body>
<form id="form1">
<button id="btn1" type="button">HTML</button>
</form>
<p>Click the "Try it" button to display the id of the form the HTML button belongs to:</p>
<button onclick="myFunction()">Try it</button>
<p id="demo"></p>
<script>
function myFunction() {
var x = document.getElementById("btn1").form.id;
document.getElementById("demo").innerHTML = x;
}
</script>
</body>
</html>