designing-tests

# Designing Tests

### When to Load

- **Trigger**: Adding tests, test strategy planning, improving coverage, setting up testing infrastructure
- **Skip**: Non-test code changes where testing is not part of the task

## Test Implementation Workflow

Copy this checklist and track progress:

```
Test Implementation Progress:
- [ ] Step 1: Identify what to test
- [ ] Step 2: Select appropriate test type
- [ ] Step 3: Write tests following templates
- [ ] Step 4: Run tests and verify passing
- [ ] Step 5: Check coverage meets targets
- [ ] Step 6: Fix any failing tests
```

## Testing Pyramid

Apply the testing pyramid for balanced coverage:

```
        /\
       /  \     E2E Tests (10%)
      /----\    - Critical user journeys
     /      \   - Slow but comprehensive
    /--------\  Integration Tests (20%)
   /          \ - Component interactions
  /------------\ - API contracts
 /              \ Unit Tests (70%)
/________________\ - Fast, isolated
                   - Business logic focus
```

## Framework Selection

### JavaScript/TypeScript

| Type        | Recommended     | Alternative      |
| ----------- | --------------- | ---------------- |
| Unit        | Vitest          | Jest             |
| Integration | Vitest + MSW    | Jest + SuperTest |
| E2E         | Playwright      | Cypress          |
| Component   | Testing Library | Enzyme           |

### Python

| Type        | Recommended                 | Alternative       |
| ----------- | --------------------------- | ----------------- |
| Unit        | pytest                      | unittest          |
| Integration | pytest + httpx              | pytest + requests |
| E2E         | Playwright                  | Selenium          |
| API         | pytest + FastAPI TestClient | -                 |

### Go

| Type        | Recommended        |
| ----------- | ------------------ |
| Unit        | testing + testify  |
| Integration | testing + httptest |
| E2E         | testing + chromedp |

## Test Structure Templates

### Unit Test

```javascript
describe("[Unit] ComponentName", () => {
  describe("methodName", () => {
    it("should [expected behavior] when [condition]", () => {
      // Arrange
      const input = createTestInput();

      // Act
      const result = methodName(input);

      // Assert
      expect(result).toEqual(expectedOutput);
    });

    it("should throw error when [invalid condition]", () => {
      expect(() => methodName(invalidInput)).toThrow(ExpectedError);
    });
  });
});
```

### Integration Test

```javascript
describe("[Integration] API /users", () => {
  beforeAll(async () => {
    await setupTestDatabase();
  });

  afterAll(async () => {
    await teardownTestDatabase();
  });

  it("should create user and return 201", async () => {
    const response = await request(app)
      .post("/users")
      .send({ name: "Test", email: "test@example.com" });

    expect(response.status).toBe(201);
    expect(response.body.id).toBeDefined();
  });
});
```

### E2E Test

```javascript
describe("[E2E] User Registration Flow", () => {
  it("should complete registration successfully", async ({ page }) => {
    await page.goto("/register");

    await page.fill('[data-testid="email"]', "new@example.com");
    await page.fill('[data-testid="password"]', "SecurePass123!");
    await page.click('[data-testid="submit"]');

    await expect(page.locator(".welcome-message")).toBeVisible();
    await expect(page).toHaveURL("/dashboard");
  });
});
```

## Coverage Strategy

### What to Cover

- ✅ Business logic (100%)
- ✅ Edge cases and error handling (90%+)
- ✅ API contracts (100%)
- ✅ Critical user paths (E2E)
- ⚠️ UI components (snapshot + interaction)
- ❌ Third-party library internals
- ❌ Simple getters/setters

### Coverage Thresholds

```json
{
  "coverageThreshold": {
    "global": {
      "branches": 80,
      "functions": 80,
      "lines": 80,
      "statements": 80
    },
    "src/core/": {
      "branches": 95,
      "functions": 95
    }
  }
}
```

## Test Data Management

### Factories/Builders

```javascript
// factories/user.js
export const userFactory = (overrides = {}) => ({
  id: faker.string.uuid(),
  name: faker.person.fullName(),
  email: faker.internet.email(),
  createdAt: new Date(),
  ...overrides,
});

// Usage
const admin = userFactory({ role: "admin" });
```

### Fixtures

```javascript
// fixtures/users.json
{
  "validUser": { "name": "Test", "email": "test@example.com" },
  "invalidUser": { "name": "", "email": "invalid" }
}
```

## Mocking Strategy

### When to Mock

- ✅ External APIs and services
- ✅ Database in unit tests
- ✅ Time/Date for determinism
- ✅ Random values
- ❌ Internal modules (usually)
- ❌ The code under test

### Mock Examples

```javascript
// API mocking with MSW
import { http, HttpResponse } from "msw";

export const handlers = [
  http.get("/api/users", () => {
    return HttpResponse.json([{ id: 1, name: "John" }]);
  }),
];

// Time mocking
vi.useFakeTimers();
vi.setSystemTime(new Date("2024-01-01"));
```

## Test Validation Loop

After writing tests, run this validation:

```
Test Validation:
- [ ] All tests pass: `npm test`
- [ ] Coverage meets thresholds: `npm test -- --coverage`
- [ ] No flaky tests (run multiple times)
- [ ] Tests are independent (order doesn't matter)
- [ ] Test names clearly describe behavior
```

If any tests fail, fix them before proceeding. If coverage is below target, add more tests for uncovered code paths.

```bash
# Run tests
npm test

# Run with coverage
npm test -- --coverage

# Run specific test file
npm test -- path/to/test.spec.ts

# Run in watch mode during development
npm test -- --watch
```