api-testing

Contract Testing with Pact

Consumer Side: Define the Contract

import { PactV3 } from '@pact-foundation/pact';

const provider = new PactV3({
  consumer: 'frontend',
  provider: 'user-api',
});

describe('User API', () => {
  it('returns user by id', async () => {
    await provider
      .given('user 1 exists')
      .uponReceiving('a request for user 1')
      .withRequest({ method: 'GET', path: '/users/1' })
      .willRespondWith({
        status: 200,
        headers: { 'Content-Type': 'application/json' },
        body: { id: 1, name: 'Alice', email: '[email protected]' },
      })
      .executeTest(async (mockServer) => {
        const res = await fetch(`${mockServer.url}/users/1`);
        const data = await res.json();
        expect(data.name).toBe('Alice');
      });
  });
});

Provider Side: Verify Against Published Pacts

import { Verifier } from '@pact-foundation/pact';

new Verifier({
  providerBaseUrl: 'http://localhost:3000',
  pactBrokerUrl: 'https://pact-broker.company.com',
  provider: 'user-api',
  providerVersion: process.env.GIT_SHA,
  stateHandlers: {
    'user 1 exists': async () => {
      await db.users.create({ id: 1, name: 'Alice' });
    },
  },
}).verifyProvider();

BAD: Testing the provider implementation in isolation without consumer contracts. Changes break consumers silently.

GOOD: Consumers define expected request/response pairs. Provider CI verifies against all published pacts. Breaking changes fail the build.

Pact Broker Workflow

1. Consumer runs tests, publishes pact to broker with pact-broker publish.
2. Provider CI pulls latest pacts with pact-broker can-i-deploy --to=production.
3. If verification passes, deploy. If it fails, coordinate with consumer team before fixing.

API Mocking Strategies

MSW for Network-Level Mocking

BAD: Mocking fetch at the module level. Tests diverge from production behavior.

jest.mock('node-fetch');
fetch.mockResolvedValue({ json: () => ({ id: 1 }) });