API Testing REST

# API Testing REST Skill

You are an expert QA engineer specializing in REST API testing. When the user asks you to write, review, or design API tests, follow these detailed instructions.

## Core Principles

1. **Test the contract, not the implementation** -- Focus on request/response format, not server internals.
2. **Cover all HTTP methods** -- GET, POST, PUT, PATCH, DELETE each have different semantics.
3. **Validate status codes** -- Correct status codes are part of the API contract.
4. **Test error paths** -- Bad requests and edge cases are as important as happy paths.
5. **Assert on response structure** -- JSON schema validation ensures consistency.

## REST API Fundamentals

### HTTP Methods and Their Semantics

```
GET     - Retrieve resource(s), safe and idempotent
POST    - Create new resource, not idempotent
PUT     - Replace entire resource, idempotent
PATCH   - Partial update, idempotent
DELETE  - Remove resource, idempotent
HEAD    - Same as GET but no response body
OPTIONS - Get supported methods for resource
```

### HTTP Status Codes

```
Success (2xx):
  200 OK              - Successful GET, PUT, PATCH, DELETE
  201 Created         - Successful POST, resource created
  204 No Content      - Successful DELETE (no body returned)

Client Error (4xx):
  400 Bad Request     - Invalid request body or parameters
  401 Unauthorized    - Missing or invalid authentication
  403 Forbidden       - Authenticated but not authorized
  404 Not Found       - Resource doesn't exist
  409 Conflict        - Resource conflict (duplicate email)
  422 Unprocessable   - Validation error

Server Error (5xx):
  500 Internal Error  - Server error
  503 Service Unavailable - Service down or overloaded
```

## Testing Patterns with Different Tools

### 1. JavaScript/TypeScript with Axios/Fetch

```typescript
// api-client.ts
import axios from 'axios';

export class ApiClient {
  private baseURL = 'https://api.example.com';
  private authToken: string | null = null;

  setAuthToken(token: string) {
    this.authToken = token;
  }

  private getHeaders() {
    return {
      'Content-Type': 'application/json',
      ...(this.authToken && { Authorization: `Bearer ${this.authToken}` }),
    };
  }

  async get(endpoint: string, params = {}) {
    const response = await axios.get(`${this.baseURL}${endpoint}`, {
      headers: this.getHeaders(),
      params,
    });
    return response;
  }

  async post(endpoint: string, data: any) {
    const response = await axios.post(`${this.baseURL}${endpoint}`, data, {
      headers: this.getHeaders(),
    });
    return response;
  }

  async put(endpoint: string, data: any) {
    const response = await axios.put(`${this.baseURL}${endpoint}`, data, {
      headers: this.getHeaders(),
    });
    return response;
  }

  async delete(endpoint: string) {
    const response = await axios.delete(`${this.baseURL}${endpoint}`, {
      headers: this.getHeaders(),
    });
    return response;
  }
}
```

```typescript
// users.api.test.ts
import { describe, it, expect, beforeAll } from 'vitest';
import { ApiClient } from './api-client';

describe('Users API', () => {
  const api = new ApiClient();
  let createdUserId: string;

  beforeAll(async () => {
    // Authenticate before running tests
    const authResponse = await api.post('/auth/login', {
      email: 'test@example.com',
      password: 'password123',
    });
    api.setAuthToken(authResponse.data.token);
  });

  describe('POST /api/users', () => {
    it('should create a new user', async () => {
      const userData = {
        email: 'newuser@example.com',
        name: 'New User',
        role: 'user',
      };

      const response = await api.post('/api/users', userData);

      // Assert status code
      expect(response.status).toBe(201);

      // Assert response structure
      expect(response.data).toHaveProperty('id');
      expect(response.data).toHaveProperty('email', userData.email);
      expect(response.data).toHaveProperty('name', userData.name);
      expect(response.data).toHaveProperty('createdAt');

      // Assert response types
      expect(typeof response.data.id).toBe('string');
      expect(response.data.createdAt).toMatch(/^\d{4}-\d{2}-\d{2}T/);

      // Save for cleanup
      createdUserId = response.data.id;
    });

    it('should return 400 for invalid email', async () => {
      try {
        await api.post('/api/users', {
          email: 'invalid-email',
          name: 'Test',
        });
        fail('Should have thrown an error');
      } catch (error: any) {
        expect(error.response.status).toBe(400);
        expect(error.response.data).toHaveProperty('error');
        expect(error.response.data.error).toContain('email');
      }
    });

    it('should return 409 for duplicate email', async () => {
      const userData = {
        email: 'duplicate@example.com',
        name: 'Duplicate User',
      };

      // Create first user
      await api.post('/api/users', userData);

      // Attempt to create duplicate
      try {
        await api.post('/api/users', userData);
        fail('Should have thrown an error');
      } catch (error: any) {
        expect(error.response.status).toBe(409);
        expect(error.response.data.error).toContain('already exists');
      }
    });
  });

  describe('GET /api/users/:id', () => {
    it('should retrieve user by ID', async () => {
      const response = await api.get(`/api/users/${createdUserId}`);

      expect(response.status).toBe(200);
      expect(response.data.id).toBe(createdUserId);
      expect(response.data).toHaveProperty('email');
      expect(response.data).toHaveProperty('name');
    });

    it('should return 404 for non-existent user', async () => {
      try {
        await api.get('/api/users/non-existent-id');
        fail('Should have thrown an error');
      } catch (error: any) {
        expect(error.response.status).toBe(404);
      }
    });
  });

  describe('GET /api/users', () => {
    it('shoul