Calculate Custom fields

i have to coded like this but no output any one advie to calculate this total

cur_frm.add_fetch("budgeted_amount", "purchased_amount", "excess_amount");
frappe.ui.form.on("budgeted_amount", "purchased_amount", function(frm, doctype, name) {
    

cur_frm.cscript.custom_validate = function(doc) {

var budgeted_amount=budgeted_amount;
var purchased_amount=purchased_amount;
var excess_amount=0;


doc.excess_amount=purchased_amount-budgeted_amount;


doc.budgeted_amount=budgeted_amount;
doc.purchased_amount=purchased_amount;

}

});