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<!DOCTYPE html>
<html>
<body>
<form id="myForm">
  <input type="number" id="myNumber" value="2">
</form>
<p>Click the button to return the id of the form the number field belongs to.</p>
<button onclick="myFunction()">Try it</button>
<p id="demo"></p>
<script>
function myFunction() {
  var x = document.getElementById("myNumber").form.id;
  document.getElementById("demo").innerHTML = x;
}
</script>
</body>
</html>