In addition to what is already suggested do some tweaks in code, set 1 =0 and if you also want to remove first empty option from second combo use j = 0
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function init(sel_type, sel_style){
for(i = 0; i <= TypeArray.length; i++){
document.product.id_type.options[i] = new Option(TypeArray[i].type, TypeArray[i].id);
if(TypeArray[i].id == sel_type)
document.product.id_type.options[i].selected = true;
}
OnChange(sel_style);
}
function OnChange(sel_style){
sel_type_index = document.product.id_type.selectedIndex;
sel_type_value = parseInt(document.product.id_type[sel_type_index].value);
for(i = document.product.id_style.length - 1; i > 0; i--)
document.product.id_style.options[i] = null;
j=1;
for(i = 0; i <= StyleArray.length; i++){
if(StyleArray[i].id_type == sel_type_value){
document.product.id_style.options[j] = new Option(StyleArray[i].style, StyleArray[i].id);
if(StyleArray[i].id == sel_style) document.product.id_style.options[j].selected = true;
j++;
}
}
}