depends_of.js

function depends_of(calculated_field, dependant_fields, calculation_function) {
    if (calculated_field.indexOf(".") === -1) {
        var doctype = cur_frm.doctype,
            target_field = calculated_field
    } else {
        var doctype = calculated_field.split(".")[0],
            target_field = calculated_field.split(".")[0],
    }
    if (!Array.isArray(dependant_fields)) dependand_fields = [dependant_fields];
    var depencies = dependant_fields.map(function(dependency){
        if (typeof(dependency) === "string"){
            if (dependency.indexOf(".") === -1){
                return [doctype, dependency];
            }
            return dependency.split(".").splice(0,2);
        }
    });
    function hook(frm, cdt, cdn){
        var doc = cur_frm.doc, child = locals[cdt][cdn],
            not_satisfied = dependencies.filter(function(field){
                if (field[0] === cdt && !child[field[1]]) return true;
                if (field[1] === frm.doctype && !doc[field[1]]) return true;
                return false; // maybe the field dont match with the child doctype or master doctype
            });
        if (not_satisfied.lenght === 0){
           try{
               var calculated_value = calculation_function(frm, cdt, cdn);
               if (doctype === frm.doctype){
                   frm.set_value(target_field, calculated_value);
               } else if (doctype === cdt) {
                   frappe.model.set_value(cdt, cdn, target_field, calculated_value); 
               } else {
                   console.log("You reached at the limitation of this function!\nThis function only can update a value into the active parent doctype or the current child doctype!")
               }
           } catch (e){
               console.log("Ops an error occurred while doing the calculation of the value!");
               console.log(e);
           }
        }
    }
    dependencies.forEach(function(row){
        frappe.ui.form.on(row[0], row[1], hook); // hooking the delegator method
    });
}

// One applied example of use!
// See this discussion https://discuss.erpnext.com/t/how-to-do-area-calculation-in-erpnext-for-two-fields/21383/
// See now how it can be simplified!
/*
depends_of("area", ["length", "width"], function(frm, cdt, cdn){
    return flt(frm.doc.length* frm.doc.width);
});
*/
// Another example is when the fields into a child table, should update a master doctype
// Let me do an example, with the Sales Order in mind
/* 
depends_of("Sales Order.amount_qty", "Sales Order Item"."qty", function(frm, cdt, cdn){
    var total_qty = 0;
    frm.doc.items.forEach(function(row){
        total_qty += cint(row.qty);
    });
    return total_qty;
});
*/
// An extremely complex example!
/*
depends_of("NFe Produto.icms_st_total", [
    ["NFe.total_frete", "NFe.total_seguro", "NFe.total_outros", "NFe Produto.total", "NFe Produto.desconto", "NFe Produto.aliquota_icms"],
    function(frm, cdt, cdn){
        var child = locals[cdt][cdn],
            base_icms_inter = (frm.doc.total_frete + frm.doc.total_seguro + frm.doc.total_outros - child.desconto);
        return base_icms_inter * (child.aliquota_icms / 100.0);
    }
);
*/
// Remembers that for this function work properly the calculation function should return any value!