┌─────────────────────────────────────────────────────────┐
│ Must the logic ALWAYS execute?                          │
│ (including imports, API calls, Server Scripts)          │
├─────────────────────────────────────────────────────────┤
│ YES → Server-side (Controller or Server Script)         │
│ NO  → What is the primary goal?                         │
│       ├── UI feedback/UX improvement → Client Script    │
│       ├── Show/hide fields → Client Script              │
│       ├── Link filters → Client Script                  │
│       ├── Data validation → BOTH (client + server)      │
│       └── Calculations → Depends on criticality         │
└─────────────────────────────────────────────────────────┘

WHAT DO YOU WANT TO ACHIEVE?
│
├─► Set link field filters
│   └── setup (once, early in lifecycle)
│
├─► Add custom buttons
│   └── refresh (after each form load/save)
│
├─► Show/hide fields based on condition
│   └── refresh + {fieldname} (both needed)
│
├─► Validation before save
│   └── validate (use frappe.throw on error)
│
├─► Action after successful save
│   └── after_save
│
├─► Calculation on field change
│   └── {fieldname}
│
├─► Child table row added
│   └── {tablename}_add
│
├─► Child table field changed
│   └── Child DocType event: {fieldname}
│
└─► One-time initialization
    └── setup or onload

frappe.ui.form.on('Sales Order', {
    refresh(frm) {
        // Initial state on form load
        frm.trigger('requires_delivery');
    },
    
    requires_delivery(frm) {
        // Toggle on checkbox change AND refresh
        frm.toggle_display('delivery_date', frm.doc.requires_delivery);
        frm.toggle_reqd('delivery_date', frm.doc.requires_delivery);
    }
});

frappe.ui.form.on('Customer', {
    setup(frm) {
        // Filter MUST be in setup for consistency
        frm.set_query('city', () => ({
            filters: {
                country: frm.doc.country || ''
            }
        }));
    },
    
    country(frm) {
        // Clear city when country changes
        frm.set_value('city', '');
    }
});

frappe.ui.form.on('Sales Invoice', {
    discount_percentage(frm) {
        calculate_totals(frm);
    }
});

frappe.ui.form.on('Sales Invoice Item', {
    qty(frm, cdt, cdn) {
        calculate_row_amount(frm, cdt, cdn);
    },
    
    rate(frm, cdt, cdn) {
        calculate_row_amount(frm, cdt, cdn);
    },
    
    amount(frm) {
        // Recalculate document total on row change
        calculate_totals(frm);
    }
});

function calculate_row_amount(frm, cdt, cdn) {
    let row = frappe.get_doc(cdt, cdn);
    frappe.model.set_value(cdt, cdn, 'amount', row.qty * row.rate);
}

function calculate_totals(frm) {
    let total = 0;
    (frm.doc.items || []).forEach(row => {
        total += row.amount || 0;
    });
    
    let discount = total * (frm.doc.discount_percentage || 0) / 100;
    frm.set_value('grand_total', total - discount);
}

frappe.ui.form.on('Sales Order', {
    async customer(frm) {
        if (!frm.doc.customer) {
            // Clear fields if customer cleared
            frm.set_value({
                customer_name: '',
                territory: '',
                credit_limit: 0
            });
            return;
        }
        
        // Fetch customer details
        let r = await frappe.db.get_value('Customer', 
            frm.doc.customer, 
            ['customer_name', 'territory', 'credit_limit']
        );
        
        if (r.message) {
            frm.set_value({
                customer_name: r.message.customer_name,
                territory: r.message.territory,
                credit_limit: r.message.credit_limit
            });
        }
    }
});

frappe.ui.form.on('Sales Order', {
    async validate(frm) {
        if (frm.doc.customer && frm.doc.grand_total) {
            let r = await frappe.call({
                method: 'myapp.api.check_credit',
                args: {
                    customer: frm.doc.customer,
                    amount: frm.doc.grand_total
                }
            });
            
            if (r.message && !r.message.allowed) {
                frappe.throw(__('Credit limit exceeded. Available: {0}', 
                    [r.message.available]));
            }
        }
    }
});

// CLIENT: frm.call invokes controller method
frm.call('calculate_taxes')
    .then(() => frm.reload_doc());

// SERVER (controller): MUST have @frappe.whitelist
class SalesInvoice(Document):
    @frappe.whitelist()
    def calculate_taxes(self):
        // complex calculation
        self.tax_amount = self.grand_total * 0.21
        self.save()