javawscript 三级菜单的实现原理

省份：	请选择山东省
城市：	==请选择==威海市德州市滨州市
区县：	==请选择乡镇==威海文登荣成乳山

省份,城市，区县数据结构：【名称，父类ID,当前ID】

 onChange="changelocation2(document.editForm.smalllocation.options[document.editForm.smalllocation.selectedIndex].value)"

根据selectedIndex取得下一级关联菜单的父类ID,

function changelocation(locationid) 
{ 
document.editForm.smalllocation.length = 0; 
// document.editForm.dlmc0.length = 0; 
document.getElementById("dlmc0").length=0; 
//document.editForm.dlmc0.options[0] = new Option('==请选择==',''); 
document.getElementById("dlmc0").options[0] = new Option('==请选择==',''); //alert(locationid); 
var locationid=locationid; 
var i; 
document.editForm.smalllocation.options[0] = new Option('==请选择==',''); 
for (i=0;i < onecount; i++) 
{ 
if (subcat[i][1] == locationid) 
{ 
document.editForm.smalllocation.options[document.editForm.smalllocation.length] = new Option(subcat[i][0], subcat[i][2]); 
} 
} 
}