​x
 
<!DOCTYPE html>
<html>
<body>
​
<p>点击下面的按钮返回该按钮所属表单的 id。</p>
​
<form id="myForm">
  <button id="myBtn" type="button" onclick="myFunction()">试一试</button>
</form>
​
<p id="demo"></p>
​
<script>
function myFunction() {
  var x = document.getElementById("myBtn").form.id;
  document.getElementById("demo").innerHTML = x;
}
</script>
​
</body>
</html>
​