feat: add POST /api/cart/promo endpoint for promo-code discount

app/api/cart/promo/route.ts

@@ -0,0 +1,34 @@
+import { NextRequest, NextResponse } from "next/server";
+import { z } from "zod";
+import { cartItemSchema, totalize } from "@/lib/cart";
+
+const RequestSchema = z.object({
+  items: z.array(cartItemSchema).min(1).max(200),
+  promoCode: z.string().max(50).nullable().optional(),
+  region: z.string().min(2).max(10),
+});
+
+export async function POST(req: NextRequest) {
+  let body: unknown;
+  try {
+    body = await req.json();
+  } catch {
+    return NextResponse.json({ error: "invalid_json" }, { status: 400 });
+  }
+
+  const parsed = RequestSchema.safeParse(body);
+  if (!parsed.success) {
+    return NextResponse.json({ error: "invalid_request" }, { status: 400 });
+  }
+
+  const { items, promoCode, region } = parsed.data;
+  const code = promoCode ?? null;
+
+  const totals = totalize(items, code, region);
+
+  return NextResponse.json({
+    totals,
+    promoCode: code,
+    promoApplied: totals.discountCents > 0,
+  });
+}

tests/promo.test.ts

@@ -0,0 +1,115 @@
+import { describe, it, expect } from 'vitest';
+import { NextRequest } from 'next/server';
+import { applyDiscount } from '../lib/cart';
+import { POST } from '../app/api/cart/promo/route';
+
+// ---------------------------------------------------------------------------
+// Pure helper: applyDiscount
+// ---------------------------------------------------------------------------
+describe('applyDiscount', () => {
+  const subtotal = 10_000; // £100.00
+
+  it('returns 0 when no code provided', () => {
+    expect(applyDiscount(subtotal, null)).toBe(0);
+  });
+
+  it('WELCOME10 applies 10% discount', () => {
+    expect(applyDiscount(subtotal, 'WELCOME10')).toBe(1_000);
+  });
+
+  it('WELCOME10 is case-insensitive', () => {
+    expect(applyDiscount(subtotal, 'welcome10')).toBe(1_000);
+  });
+
+  it('VIP25 applies 25% when subtotal >= 10000', () => {
+    expect(applyDiscount(10_000, 'VIP25')).toBe(2_500);
+  });
+
+  it('VIP25 returns 0 when subtotal < 10000', () => {
+    expect(applyDiscount(9_999, 'VIP25')).toBe(0);
+  });
+
+  it('FREESHIP returns 0 (shipping handled elsewhere)', () => {
+    expect(applyDiscount(subtotal, 'FREESHIP')).toBe(0);
+  });
+
+  it('unknown code returns 0', () => {
+    expect(applyDiscount(subtotal, 'NOTACODE')).toBe(0);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// POST /api/cart/promo — route handler
+// ---------------------------------------------------------------------------
+
+function buildRequest(body: unknown): NextRequest {
+  return new NextRequest('http://localhost/api/cart/promo', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(body),
+  });
+}
+
+describe('POST /api/cart/promo', () => {
+  const singleItem = [{ productId: 'p1', quantity: 2, unitPriceCents: 1_000 }];
+
+  it('returns 400 for non-JSON body', async () => {
+    const req = new NextRequest('http://localhost/api/cart/promo', {
+      method: 'POST',
+      headers: { 'Content-Type': 'text/plain' },
+      body: 'not json',
+    });
+    const res = await POST(req);
+    expect(res.status).toBe(400);
+    const json = await res.json();
+    expect(json.error).toBe('invalid_json');
+  });
+
+  it('returns 400 when items array is empty', async () => {
+    const res = await POST(buildRequest({ items: [], region: 'GB' }));
+    expect(res.status).toBe(400);
+    const json = await res.json();
+    expect(json.error).toBe('invalid_request');
+  });
+
+  it('returns 400 when region is missing', async () => {
+    const res = await POST(buildRequest({ items: singleItem }));
+    expect(res.status).toBe(400);
+  });
+
+  it('applies WELCOME10 promo code correctly', async () => {
+    const res = await POST(buildRequest({ items: singleItem, promoCode: 'WELCOME10', region: 'GB' }));
+    expect(res.status).toBe(200);
+    const json = await res.json();
+    // subtotal = 2 * 1000 = 2000; discount = 200; taxable = 1800; tax GB 20% = 360; total = 2160
+    expect(json.totals.subtotalCents).toBe(2_000);
+    expect(json.totals.discountCents).toBe(200);
+    expect(json.totals.taxCents).toBe(360);
+    expect(json.totals.totalCents).toBe(2_160);
+    expect(json.promoApplied).toBe(true);
+    expect(json.promoCode).toBe('WELCOME10');
+  });
+
+  it('sets promoApplied=false for unknown code', async () => {
+    const res = await POST(buildRequest({ items: singleItem, promoCode: 'BOGUS', region: 'GB' }));
+    expect(res.status).toBe(200);
+    const json = await res.json();
+    expect(json.totals.discountCents).toBe(0);
+    expect(json.promoApplied).toBe(false);
+  });
+
+  it('works without a promo code (omitted)', async () => {
+    const res = await POST(buildRequest({ items: singleItem, region: 'US-CA' }));
+    expect(res.status).toBe(200);
+    const json = await res.json();
+    expect(json.promoCode).toBeNull();
+    expect(json.promoApplied).toBe(false);
+  });
+
+  it('works with promoCode: null explicitly', async () => {
+    const res = await POST(buildRequest({ items: singleItem, promoCode: null, region: 'DE' }));
+    expect(res.status).toBe(200);
+    const json = await res.json();
+    expect(json.promoApplied).toBe(false);
+  });
+});

vitest.config.ts

@@ -1,6 +1,12 @@
 import { defineConfig } from 'vitest/config';
+import path from 'path';
 
 export default defineConfig({
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, '.'),
+    },
+  },
   test: {
     environment: 'node',
     include: ['tests/**/*.test.ts'],