test(product): add cart behavior tests for ProductDetailPage

Covers: Add to Cart, Add to Cart twice (quantity increment), Buy Now
standalone, and Add to Cart followed by Buy Now (no duplication).

test/components/storefront/ProductDetailPage.test.tsx

@@ -0,0 +1,90 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { render, screen, fireEvent, waitFor } from '@testing-library/react'
+import ProductDetailPage from '@/app/products/[slug]/page'
+
+const mockPush = vi.fn()
+
+vi.mock('next/navigation', () => ({
+  useParams: () => ({ slug: 'test-product' }),
+  useRouter: () => ({ push: mockPush }),
+}))
+
+vi.mock('@/components/storefront/Navbar', () => ({
+  Navbar: () => null,
+}))
+
+const mockProduct = {
+  product: {
+    id: 'prod-1',
+    title: 'Test Product',
+    slug: 'test-product',
+    description: 'A test product',
+    basePrice: 1000,
+    currency: 'EUR',
+    stock: 10,
+    images: [],
+    variants: [],
+    categories: [],
+    reviews: [],
+  },
+}
+
+describe('ProductDetailPage - cart behavior', () => {
+  beforeEach(() => {
+    localStorage.clear()
+    mockPush.mockClear()
+    vi.stubGlobal(
+      'fetch',
+      vi.fn().mockResolvedValue({ json: () => Promise.resolve(mockProduct) })
+    )
+  })
+
+  it('Add to Cart aggiunge 1 articolo al carrello', async () => {
+    render(<ProductDetailPage />)
+    await waitFor(() => screen.getByText('Test Product'))
+
+    fireEvent.click(screen.getByText('Add to Cart'))
+
+    const cart = JSON.parse(localStorage.getItem('cart') || '[]')
+    expect(cart).toHaveLength(1)
+    expect(cart[0].quantity).toBe(1)
+    expect(cart[0].productId).toBe('prod-1')
+  })
+
+  it('Add to Cart cliccato due volte incrementa la quantità a 2', async () => {
+    render(<ProductDetailPage />)
+    await waitFor(() => screen.getByText('Test Product'))
+
+    fireEvent.click(screen.getByText('Add to Cart'))
+    fireEvent.click(screen.getByText('Add to Cart'))
+
+    const cart = JSON.parse(localStorage.getItem('cart') || '[]')
+    expect(cart).toHaveLength(1)
+    expect(cart[0].quantity).toBe(2)
+  })
+
+  it('Buy Now aggiunge 1 articolo e naviga al carrello', async () => {
+    render(<ProductDetailPage />)
+    await waitFor(() => screen.getByText('Test Product'))
+
+    fireEvent.click(screen.getByText('Buy Now'))
+
+    const cart = JSON.parse(localStorage.getItem('cart') || '[]')
+    expect(cart).toHaveLength(1)
+    expect(cart[0].quantity).toBe(1)
+    expect(mockPush).toHaveBeenCalledWith('/cart')
+  })
+
+  it('Add to Cart seguito da Buy Now non duplica: rimane 1 articolo con quantità 1', async () => {
+    render(<ProductDetailPage />)
+    await waitFor(() => screen.getByText('Test Product'))
+
+    fireEvent.click(screen.getByText('Add to Cart'))
+    fireEvent.click(screen.getByText('Buy Now'))
+
+    const cart = JSON.parse(localStorage.getItem('cart') || '[]')
+    expect(cart).toHaveLength(1)
+    expect(cart[0].quantity).toBe(1)
+    expect(mockPush).toHaveBeenCalledWith('/cart')
+  })
+})