Web Forms API

<!DOCTYPE html>
<html>
<body>

<form>
  First Name: <input type="text" name="fname" value="Donald"><br>
  Last Name: <input type="text" name="lname" value="Duck">
</form>

<p>Click the button to display the number of form elements in the document.</p>

<button onclick="myFunction()">Try it</button>

<p id="demo"></p>

<script>
function myFunction() {
  var x = document.forms.length;
  document.getElementById("demo").innerHTML = x;
}
</script>

</body>
</html>

Click the button to display the number of form elements in the document.