JavaScript获取表单内元素数量的方法

<!DOCTYPE html>
<html>
<body>
<form id="frm1" action="form_action.asp">
First name: <input type="text" name="fname" value="Donald"><br>
Last name: <input type="text" name="lname" value="Duck"><br>
<input type="submit" value="Submit">
</form>
<p>The number of elements in "frm1" are:
<script>
document.write(document.getElementById("frm1").length);
</script>
</p>
</body>
</html>