例子，js获取下拉列表框元素个数。
复制代码 代码示例:
<!DOCTYPE html>
<html>
<head>
<script>
function getLength()
{
alert(document.getElementById("mySelect").length);//元素个数
}
</script>
</head>
<body>
<form>
<select id="mySelect">
<option>Apple</option>
<option>Pear</option>
<option>Banana</option>
<option>Orange</option>
</select>
<input type="button" onclick="getLength()" value="How many options in the list?">
</form>
</body>
</html>
