asp.net中JavaScript数据验证实现代码

DDLmaterial.Attributes.Add("onchange", "alert('注意：您修改了物品类型!')");

window.onload = function() {
var ddl= document.getElementById('DDLmaterial');
var first =ddl.options[ddl.selectedIndex].text;
}

<script type ="text/javascript" >
var first;
var ddl;
window.onload = function() {
ddl= document.getElementById('DDLmaterial');
first =ddl.options[ddl.selectedIndex].text;
}
function changeAlert() {
var sencond = ddl.options[ddl.selectedIndex].text;
if (sencond != first) {
alert("注意：您修改了物品类型!");
}
}
</script>