下载了一个asp.net网站，有一点看不懂，请高手指教！！！
这是一个用户注册信息检测的脚本函数：


function checkform()
{
var f= document.getElementById( "aspnetForm ");
if(trim(f.ctl00_ContentPlaceHolder1_textboxusername.value)== " ")
{
alert ( "请输入用户名 ");
f.ctl00_ContentPlaceHolder1_textboxusername.value= " ";
f.ctl00_ContentPlaceHolder1_textboxusername.focus();
return false;
}
else if(!checkusername(trim(f.ctl00_ContentPlaceHolder1_textboxusername.value)))
{
alert ( "您输入的用户名的格式错误 ");
//f.ctl00_ContentPlaceHolder1_textboxusername.value= " ";
f.ctl00_ContentPlaceHolder1_textboxusername.focus();
return false;
}
if(trim(f.ctl00_ContentPlaceHolder1_TextBoxpassword1.value)== " ")
{
alert ( "密码不能为空!!!!!!!!!!!!! ");
f.ctl00_ContentPlaceHolder1_TextBoxpassword1.focus();
return false;
}
if(f.ctl00_ContentPlaceHolder1_TextBoxpassword1.value!=f.ctl00_ContentPlaceHolder1_TextBoxpassword2.value)
{
alert ( "两次输入密码不一致 ");
f.ctl00_ContentPlaceHolder1_TextBoxpassword1.value= " ";
f.ctl00_ContentPlaceHolder1_TextBoxpassword2.value= " ";
f.ctl00_ContentPlaceHolder1_TextBoxpassword1.focus();
return false;
}
if(trim(f.ctl00_ContentPlaceHolder1_TextBoxemail.value)== " ")
{
alert ( "请输入邮箱地址 ");
f.ctl00_ContentPlaceHolder1_TextBoxemail.value= " ";
f.ctl00_ContentPlaceHolder1_TextBoxemail.focus();
return false;
}
else if(!isemail(trim(f.ctl00_ContentPlaceHolder1_TextBoxemail.value)))
{
alert ( "您输入的邮箱的格式错误 ");
//f.ctl00_ContentPlaceHolder1_textboxusername.value= " ";
f.ctl00_ContentPlaceHolder1_TextBoxemail.focus();

return false;
}
return true;

}


大家看这句：var f= document.getElementById( "aspnetForm ");
这里这个aspnetform我再整个项目中搜了5遍，没找到！！是系统定义的吗？？
我下文有个form1,改了之后还不能用。


f.ctl00_ContentPlaceHolder1_TextBoxemail.focus();
还有这句，这其中的ctl00有是什么东西，也找不到，JS使用的是什么引用方法？？
哪位能给我解释下？？

[解决办法]
定义成了用户控件
这些名字是系统自已生成的