InvoiceSummary
The InvoiceSummary interface represents the calculated totals for an invoice, including subtotals, discounts, tax, and final total.
Sum of all line items before discounts and tax (quantity × rate)
Total amount of item-level discounts applied
Subtotal minus item discounts, before tax
Total tax amount calculated on the discounted subtotal
Final invoice total (subtotalAfterDiscount + taxAmount)
TypeScript definition
From app/lib/types.ts:56-62:
export interface InvoiceSummary {
subtotal: number;
itemDiscount: number;
subtotalAfterDiscount: number;
taxAmount: number;
total: number;
}
Calculation logic
The summary is calculated using the calculateInvoiceSummary function from app/lib/invoice.ts:
export function calculateInvoiceSummary(
items: InvoiceItem[],
globalTaxRate: number = 0,
): InvoiceSummary {
let subtotal = 0;
let itemDiscount = 0;
let taxAmount = 0;
// Calculate subtotal and item-level discounts
for (const item of items) {
const qty = item.quantity || 0;
const rate = item.rate || 0;
const itemTotal = qty * rate;
const discount = item.discount ? (itemTotal * item.discount) / 100 : 0;
const itemTaxBase = itemTotal - discount;
subtotal += itemTotal;
itemDiscount += discount;
// Apply item-level tax rate if provided, otherwise use global rate
const taxRate = item.taxRate ?? globalTaxRate;
taxAmount += (itemTaxBase * taxRate) / 100;
}
const subtotalAfterDiscount = subtotal - itemDiscount;
const total = subtotalAfterDiscount + taxAmount;
return {
subtotal,
itemDiscount,
subtotalAfterDiscount,
taxAmount,
total,
};
}
Example
Given these invoice items:
const items: InvoiceItem[] = [
{
id: 'item_1',
description: 'Web Design',
quantity: 10,
rate: 100,
discount: 10, // 10% discount
taxRate: 20 // 20% VAT
},
{
id: 'item_2',
description: 'Hosting',
quantity: 1,
rate: 50,
taxRate: 20
}
];
const summary = calculateInvoiceSummary(items);
{
"subtotal": 1050.00,
"itemDiscount": 100.00,
"subtotalAfterDiscount": 950.00,
"taxAmount": 190.00,
"total": 1140.00
}
- Item 1: (10 × £100) = £1000, minus 10% (£100) = £900 + 20% VAT (£180) = £1080
- Item 2: (1 × £50) = £50 + 20% VAT (£10) = £60
- Total: £1140
Usage in components
import { calculateInvoiceSummary } from '@/app/lib/invoice';
import type { InvoiceSummary, InvoiceItem } from '@/app/lib/types';
// Calculate summary for display
const items: InvoiceItem[] = [/* ... */];
const globalTaxRate = 20; // 20% VAT
const summary: InvoiceSummary = calculateInvoiceSummary(items, globalTaxRate);
console.log(`Subtotal: ${summary.subtotal}`);
console.log(`Discount: -${summary.itemDiscount}`);
console.log(`Tax: ${summary.taxAmount}`);
console.log(`Total: ${summary.total}`);
The tax calculation applies to the discounted amount, not the original subtotal. Item-level tax rates take precedence over the global invoice tax rate.