form 属性可返回对包含选项的表单的引用。
optionObject.form
<html>
<body>
<form id="myForm">
<select id="mySelect">
<option id="apple">Apple</option>
<option id="orange">Orange</option>
<option id="pineapple">Pineapple</option>
<option id="banana">Banana</option>
</select>
</form>
<p>The id of the form is:
<script type="text/javascript">
document.write(document.getElementById("apple").form.id)
</script>
</p>
</body>
</html>