​x
 
<!DOCTYPE html>
<html>
<body>
​
<form id="form1">
<button id="btn1" type="button">HTML</button>
</form>
​
<p>单击“试一试”按钮以显示 HTML 按钮所属表单的 id：</p>
​
<button onclick="myFunction()">试一试</button>
​
<p id="demo"></p>
​
<script>
function myFunction() {
  var x = document.getElementById("btn1").form.id;
  document.getElementById("demo").innerHTML = x;
}
</script>
​
</body>
</html>
​