integration-test

name	integration-test
description	Write integration tests with Jest for API routes and database operations. Use when testing backend logic, API endpoints, or data layer.

Integration Test (Jest)

Standard Structure

src/__tests__/integration/
├── books.test.ts         # All book-related API tests
├── categories.test.ts    # All category API tests
├── auth.test.ts          # Authentication API tests
└── cart.test.ts          # Cart/checkout API tests

Naming Convention

• File: {feature}.test.ts (e.g., featured-books.test.ts, categories.test.ts)
• Describe: {Feature} API (e.g., Books API, Categories API)
• Nested describe: {HTTP_METHOD} /api/{route} (e.g., GET /api/books/featured)

One File Per Feature

Consolidate ALL related API tests into ONE file:

//  CORRECT: One file for all book APIs
// src/__tests__/integration/books.test.ts
describe('Books API', () => {
  describe('GET /api/books/featured', () => {...});
  describe('GET /api/books/bestsellers', () => {...});
});

// WRONG: Multiple files for same feature
// featured-books.test.ts
// bestsellers.test.ts

---

⭐ Branch Coverage Strategy (3-4 tests per API)

Focus on BRANCH COVERAGE, not quantity! Each API needs only 3 tests:

Branch	Test Case	Priority
Happy path	Valid request → success data	⭐⭐⭐ MUST
Empty case	No data → empty array	⭐⭐⭐ MUST
Error case	DB error → error response	⭐⭐ SHOULD

Example: 3 Tests = 100% Branch Coverage

describe('GET /api/books/featured', () => {
  beforeEach(() => jest.clearAllMocks());

  //  Branch 1: Happy path
  it('returns books when data exists', async () => {
    (prisma.book.findMany as jest.Mock).mockResolvedValue([
      { id: '1', title: 'Book 1' }
    ]);
    const response = await GET();
    const data = await response.json();
    expect(data.success).toBe(true);
    expect(data.data).toHaveLength(1);
  });

  //  Branch 2: Empty case
  it('returns empty array when no books', async () => {
    (prisma.book.findMany as jest.Mock).mockResolvedValue([]);
    const response = await GET();
    const data = await response.json();
    expect(data.success).toBe(true);
    expect(data.data).toEqual([]);
  });

  //  Branch 3: Error case
  it('handles database error', async () => {
    (prisma.book.findMany as jest.Mock).mockRejectedValue(new Error('DB Error'));
    const response = await GET();
    const data = await response.json();
    expect(data.success).toBe(false);
  });
});
// Total: 3 tests covering ALL branches!

Anti-pattern: Redundant Tests

// WRONG: Multiple tests for same branch (happy path)
it('returns featured books', async () => {...});     // Happy path
it('returns books with correct fields', async () => {...}); // Same branch!
it('returns books sorted by date', async () => {...});      // Same branch!
it('returns max 10 books', async () => {...});              // Same branch!
// 4 tests but only 1 branch covered!

---

⭐ FLEXIBLE ASSERTIONS (Avoid Hardcoded Values)

NEVER hardcode calculated values - they cause mock-assertion mismatch!

// WRONG - Hardcoded value causes failures
const mockReviews = [{ rating: 5 }, { rating: 4 }];  // avg = 4.5
expect(data.averageRating).toBe(5);  // FAILS! Mock gives 4.5, not 5

// WRONG - Hardcoded specific values
expect(data.data[0].price).toBe(29.99);
expect(data.data[0].totalSold).toBe(100);
expect(data.data[0].averageRating).toBe(4.5);

Use Flexible Matchers Instead:

//  Check structure exists (RECOMMENDED)
expect(data.data[0]).toMatchObject({
  id: expect.any(String),
  title: expect.any(String),
  price: expect.any(Number),
  averageRating: expect.any(Number),
});

//  Check array has items
expect(data.data).toHaveLength(1);  // or toBeGreaterThan(0)

//  Check value ranges (not exact)
expect(data.data[0].averageRating).toBeGreaterThanOrEqual(0);
expect(data.data[0].averageRating).toBeLessThanOrEqual(5);

//  Check field exists
expect(data.data[0].totalSold).toBeDefined();
expect(data.data[0].category).toBeDefined();

//  Check boolean/success flags (OK to be exact)
expect(data.success).toBe(true);

Quick Reference:

Instead of	Use
.toBe(4.5)	.toBeGreaterThanOrEqual(0)
.toBe(100)	.toBeDefined() or expect.any(Number)
.toBe('exact string')	.toContain('substring') or expect.any(String)
.toEqual({...exact...})	.toMatchObject({...partial...})

---

⛔ CRITICAL: NEVER CHECK response.status

NextResponse.status is ALWAYS undefined in Jest environment. This is a known limitation.

// ⛔ WRONG - WILL ALWAYS FAIL (response.status is undefined)
const response = await GET(request);
expect(response.status).toBe(200);  // FAILS: Expected 200, Received: undefined

//  CORRECT - Check data.success or data properties instead
const response = await GET(request);
const data = await response.json();
expect(data.success).toBe(true);     // Check API response structure
expect(data.data).toBeDefined();     // Check data exists
expect(data.error).toBeUndefined();  // No error for success

//  For error cases - Check error response structure
const response = await POST(invalidRequest);
const data = await response.json();
expect(data.success).toBe(false);    // API returns success: false
expect(data.error).toBeDefined();    // Error message exists

WHY: When calling Next.js route handlers directly (not via HTTP), the Response object's .status property is not populated correctly in Jest's mock environment.

⚠️ CRITICAL RULES - READ FIRST

• DO NOT create config files (jest.config.*, tsconfig.json)
• Config files ALREADY EXIST in project - use them as-is
• ONLY create TEST files: *.test.ts
• READ SOURCE CODE FIRST - Check actual exports, types, function signatures before writing tests
• DO NOT INVENT APIs - Only test routes/functions that actually exist in the codebase
• NEVER use response.status - it's always undefined (see above)

⚠️ Response Methods in Jest

In Jest mock environment, use response.json() directly - do NOT use response.text().

// WRONG - response.text() is not available in Jest mock
const response = await GET(request);
const text = await response.text();  // ERROR: response.text is not a function

// WRONG - response.status is undefined
const response = await GET(request);
expect(response.status).toBe(200);   // FAILS: undefined !== 200

//  CORRECT - Use response.json() and check data properties
const response = await GET(request);
const data = await response.json();
expect(data.success).toBe(true);     // Check success flag
expect(data.data).toEqual(expected); // Check actual data

BEST PRACTICE: For integration tests, mock at the database layer (Prisma) and check data.success or data.error instead of response.status.

⚠️ API Response Wrapper - UNDERSTAND THE FLOW

This project uses successResponse() helper that wraps data. Understand this flow:

// 1. Prisma mock returns RAW data (what database would return)
mockFindMany.mockResolvedValue([
  { id: '1', title: 'Book 1', author: 'Author 1' },
  { id: '2', title: 'Book 2', author: 'Author 2' },
]);

// 2. Route handler calls Prisma, gets raw data, wraps using successResponse()
// Inside route.ts: return successResponse(books);
// This returns: { success: true, data: [{id: '1'...}, {id: '2'...}] }

// 3. Test asserts on WRAPPED response structure
const response = await GET(request);
const data = await response.json();

//  CORRECT - Check wrapped response
expect(data.success).toBe(true);           // Check success flag
expect(data.data).toHaveLength(2);         // Check data array length
expect(data.data[0].title).toBe('Book 1'); // Check data content

// ⛔ WRONG - Checking as if raw array (COMMON MISTAKE!)
expect(data).toHaveLength(2);              // FAILS! data is object, not array
expect(data[0].title).toBe('Book 1');      // FAILS! data[0] is undefined
expect(data.length).toBe(2);               // FAILS! data.length is undefined

REMEMBER: data is always { success: boolean, data: T }, never raw array!

⚠️ GET Routes with Query Parameters (searchParams)

Many Next.js routes use request.nextUrl.searchParams. You MUST use NextRequest:

import { NextRequest } from 'next/server';
import { GET } from '@/app/api/books/search/route';

it('searches books by query', async () => {
  //  CORRECT - Create NextRequest with URL containing query params
  const request = new NextRequest('http://localhost/api/books/search?q=math&page=1');
  
  const response = await GET(request);
  const data = await response.json();
  
  expect(data.success).toBe(true);
  expect(data.data).toBeDefined();
});

// WRONG - Basic Request doesn't have nextUrl property
const request = new Request('http://localhost/api/search');
// In route.ts: request.nextUrl.searchParams.get('q')
// ERROR: Cannot read property 'searchParams' of undefined!

// WRONG - Missing query params in URL
const request = new NextRequest('http://localhost/api/search');
// route expects ?q=... but URL has no params → validation error or empty results

RULE: If route uses request.nextUrl.searchParams, you MUST use NextRequest with query params in URL.

⚠️ TYPESCRIPT STRICT RULES

//  CORRECT - Explicit types for ALL parameters
const mockFn = jest.fn((...args: unknown[]) => mockImpl(...args));
const handler = (req: Request, params: { id: string }) => {...};

// WRONG - Implicit any (will cause TS errors)
const mockFn = jest.fn((...args) => mockImpl(...args));  // Error: implicit any
const handler = (req, params) => {...};  // Error: implicit any

⚠️ IMPORT RULES

//  CORRECT imports
import { getServerSession } from 'next-auth';           // Named import
import { prisma } from '@/lib/prisma';                  // Named import
import { GET, POST } from '@/app/api/users/route';      // Named imports for route handlers

// WRONG imports - DO NOT USE
import getServerSession from 'next-auth';               // Wrong: not default export
import prisma from '@/lib/prisma';                      // Wrong: check actual export

When to Use

• Testing API route handlers (GET, POST, PUT, DELETE)
• Testing database operations (Prisma)
• Testing service layer logic
• Testing with mocked external services

File Location (FIXED - DO NOT CHANGE)

src/__tests__/integration/     # Integration tests (Jest)

IMPORTANT: Always use src/__tests__/integration/ for integration tests. Do NOT use other folders like __tests__/integration/ or tests/.

Test Structure

import { POST } from '@/app/api/users/route';

// Mock functions at top level (outside describe)
const mockCreate = jest.fn();
const mockFindMany = jest.fn();
const mockFindUnique = jest.fn();

jest.mock('@/lib/prisma', () => ({
  prisma: {
    user: {
      create: (...args: unknown[]) => mockCreate(...args),
      findMany: (...args: unknown[]) => mockFindMany(...args),
      findUnique: (...args: unknown[]) => mockFindUnique(...args),
    },
  },
}));

describe('Story: {story_title}', () => {
  beforeEach(() => {
    jest.clearAllMocks();
  });

  describe('POST /api/users', () => {
    it('creates user with valid data', async () => {
      // Arrange
      const mockUser = { id: 'test-1', name: 'Test User', email: '[e-mail]' };
      mockCreate.mockResolvedValue(mockUser);
      
      // Act
      const request = new Request('http://localhost/api/users', {
        method: 'POST',
        body: JSON.stringify({ name: 'Test User', email: '[e-mail]' }),
      });
      const response = await POST(request);
      const data = await response.json();
      
      // Assert - Check data properties, NOT response.status
      expect(data.success).toBe(true);
      expect(data.data).toEqual(mockUser);
      expect(mockCreate).toHaveBeenCalledWith({
        data: { name: 'Test User', email: '[e-mail]' },
      });
    });

    it('returns error for invalid data', async () => {
      // Arrange - empty body
      const request = new Request('http://localhost/api/users', {
        method: 'POST',
        body: JSON.stringify({}),
      });
      
      // Act
      const response = await POST(request);
      const data = await response.json();
      
      // Assert - Check error response, NOT response.status
      expect(data.success).toBe(false);
      expect(data.error).toBeDefined();
    });
  });
});

Mock Patterns

Prisma Mock Setup

// At top of file, outside describe
const mockFindUnique = jest.fn();
const mockCreate = jest.fn();
const mockUpdate = jest.fn();
const mockDelete = jest.fn();
const mockFindMany = jest.fn();

jest.mock('@/lib/prisma', () => ({
  prisma: {
    user: {
      findUnique: (...args: unknown[]) => mockFindUnique(...args),
      create: (...args: unknown[]) => mockCreate(...args),
      update: (...args: unknown[]) => mockUpdate(...args),
      delete: (...args: unknown[]) => mockDelete(...args),
      findMany: (...args: unknown[]) => mockFindMany(...args),
    },
  },
}));

Fetch Mock

global.fetch = jest.fn();

// In test
(fetch as jest.Mock).mockResolvedValue({
  ok: true,
  json: async () => ({ data: 'test' }),
});

NextAuth Session Mock

jest.mock('next-auth', () => ({
  getServerSession: jest.fn(),
}));

// In test
(getServerSession as jest.Mock).mockResolvedValue({
  user: { id: 'user-1', email: '[e-mail]' },
});

AAA Pattern (Arrange-Act-Assert)

1. Arrange: Set up test data, mocks, preconditions
2. Act: Execute the code being tested
3. Assert: Verify expected outcomes

Commands

pnpm test                           # All tests
pnpm test tests/integration         # Integration only
pnpm test --coverage                # With coverage
pnpm test --watch                   # Watch mode

Important Rules

1. NO ESM packages - Don't use uuid, nanoid. Use hardcoded IDs: "test-id-123"
2. Clear mocks - Always jest.clearAllMocks() in beforeEach
3. Async/await - Always await async operations
4. Isolation - Each test should be independent

Anti-Patterns - DO NOT DO

Don't create helper functions for response extraction

// WRONG - Unnecessary abstraction, prone to errors
async function extractResponse(response: Response) {
  const data = await response.json();
  return { status: response.status, data };
}
const { status, data } = await extractResponse(response);

//  CORRECT - Use directly, simple and clear
const response = await GET(request);
const data = await response.json();
expect(response.status).toBe(200);

Don't check exact Prisma query structure

// WRONG - Brittle, breaks on any refactor
expect(mockFindMany).toHaveBeenCalledWith({
  where: { OR: [{ title: { contains: 'test', mode: 'insensitive' } }] },
  skip: 0,
  take: 20,
  include: { category: { select: { id: true, name: true } } },
  orderBy: [{ featured: 'desc' }, { createdAt: 'desc' }],
});

//  CORRECT - Check behavior via response data
expect(mockFindMany).toHaveBeenCalled();
expect(data).toHaveLength(2);
expect(data[0].title).toBe('Expected Title');

Don't use Date objects in mock data

// WRONG - Date comparison issues in assertions
const mockBook = {
  id: 'book-1',
  createdAt: new Date('2023-01-15'),
  updatedAt: new Date('2023-01-15'),
};

//  CORRECT - Use ISO strings for dates
const mockBook = {
  id: 'book-1',
  createdAt: '2023-01-15T00:00:00.000Z',
  updatedAt: '2023-01-15T00:00:00.000Z',
};

Keep mock data minimal

// WRONG - Too many fields, hard to maintain
const mockUser = {
  id: 'user-1',
  name: 'Test User',
  email: '[e-mail]',
  phone: '123-456-7890',
  address: '123 Main St',
  city: 'Test City',
  country: 'Test Country',
  createdAt: '2023-01-01',
  updatedAt: '2023-01-01',
  role: 'user',
  isActive: true,
  // ... more fields
};

//  CORRECT - Only include fields being tested
const mockUser = { id: 'user-1', name: 'Test User' };

References

• mock-patterns.md - Advanced mocking patterns for Prisma, Auth, External APIs