API Testing

Jest · Reference cheat sheet

📋 Overview

Test fetch/axios clients with mocked fetch, MSW, or against a local server. Assert status, JSON shape, and error mapping.

🔧 Core concepts

Approach	When
Mock `global.fetch`	Unit the client
MSW	Realistic HTTP at boundary
Supertest / real URL	Integration
`nock`	Node outbound mocks

💡 Examples

Mock fetch:

afterEach(() => {
  jest.restoreAllMocks();
});

test('getUser maps json', async () => {
  global.fetch = jest.fn().mockResolvedValue({
    ok: true,
    status: 200,
    json: async () => ({ id: 1, name: 'Ada' }),
  });

  const { getUser } = require('./api');
  await expect(getUser(1)).resolves.toEqual({ id: 1, name: 'Ada' });
  expect(fetch).toHaveBeenCalledWith('/api/users/1', expect.any(Object));
});

Integration with fetch + base URL:

const BASE = process.env.API_URL || 'http://127.0.0.1:8000';

test('health', async () => {
  const res = await fetch(`${BASE}/health`);
  expect(res.status).toBe(200);
  await expect(res.json()).resolves.toMatchObject({ ok: true });
});

Expect 401:

test('me without token', async () => {
  const res = await fetch(`${BASE}/api/me`);
  expect(res.status).toBe(401);
});

⚠️ Pitfalls

Reset mocks between tests (clearAllMocks / restoreAllMocks).
Absolute URLs in Node 18+ fetch need a running server or MSW.

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

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