test-driven-development

# Test-Driven Development ## The Red-Green-Refactor Cycle ``` RED → Write a failing test that describes the desired behavior GREEN → Write the minimum code to make it pass (no more, no less) REFACTOR → Improve the code without changing behavior; tests stay green ``` Never skip the RED step. If you write code before the test, you lose confidence that your test actually tests anything. ## Test Structure: AAA Pattern ```typescript // Arrange → Act → Assert — every test follows this shape describe('UserService', () => { describe('create', () => { it('should hash the password before saving', async () => { // ARRANGE — set up everything the test needs const dto: CreateUserDto = { email: 'jane@example.com', password: 'S3cureP@ss!', name: 'Jane Doe', } const mockRepo = createMockUserRepo() const mockHasher = createMockHasher({ result: 'hashed_password' }) const sut = new UserService(mockRepo, mockHasher) // ACT — call the thing under test await sut.create(dto) // ASSERT — verify outcomes expect(mockHasher.hash).toHaveBeenCalledWith('S3cureP@ss!') expect(mockRepo.save).toHaveBeenCalledWith( expect.objectContaining({ passwordHash: 'hashed_password' }) ) }) }) }) ``` ## Vitest Setup ```typescript // vitest.config.ts import { defineConfig } from 'vitest/config' import tsconfigPaths from 'vite-tsconfig-paths' export default defineConfig({ plugins...