Implementation principle of javawscript three-level menu_javascript skills

The code is as follows:


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('==Please select==','');
document.getElementById("dlmc0") .options[0] = new Option('==Please select==','');

//alert(locationid);
var locationid=locationid;
var i;
document.editForm.smalllocation.options[0] = new Option('==Please select==','');
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]);
}
}