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<!DOCTYPE html>
<html>
<body>
<form id="myForm">
First name: <input type="text" name="fname">
<input type="submit" id="mySubmit">
</form>
<p>Click the "Try it" button to display the id of the form the Submit button belongs to.</p>
<button onclick="myFunction()">Try it</button>
<p id="demo"></p>
<script>
function myFunction() {
  var x = document.getElementById("mySubmit").form.id;
  document.getElementById("demo").innerHTML = x;
}
</script>
</body>
</html>