How to generate dependency-injection-friendly code with Cursor

1---
2description: Dependency injection rules
3globs: "src/services/**/*.ts,src/repositories/**/*.ts"
4alwaysApply: true
5---
6
7## Dependency Injection Rules
8- ALWAYS depend on interfaces, NEVER on concrete classes
9- ALWAYS accept dependencies through the constructor
10- NEVER use 'new' to instantiate dependencies inside a class
11- Define interfaces in src/types/interfaces.ts
12- Use @injectable() decorator from tsyringe
13- Use @inject() for constructor parameters
14- Name interfaces with I prefix: IUserRepository, IOrderService
15- Export interfaces separately from implementations
16- Every class must be testable with mocked dependencies

1// Cursor Chat prompt (Cmd+L):
2// Create interfaces for a UserService and UserRepository
3// in src/types/interfaces.ts. UserRepository should have
4// findById, findAll, create, update, delete methods.
5// UserService should have getUser, listUsers, createUser,
6// updateUser. All methods return Promises with typed results.
7
8export interface IUserRepository {
9  findById(id: string): Promise<User | null>;
10  findAll(page: number, limit: number): Promise<User[]>;
11  create(data: CreateUserDto): Promise<User>;
12  update(id: string, data: UpdateUserDto): Promise<User>;
13  delete(id: string): Promise<void>;
14}
15
16export interface IUserService {
17  getUser(id: string): Promise<User>;
18  listUsers(page: number, limit: number): Promise<User[]>;
19  createUser(data: CreateUserDto): Promise<User>;
20  updateUser(id: string, data: UpdateUserDto): Promise<User>;
21}

1// Cursor Chat prompt (Cmd+L):
2// @src/types/interfaces.ts Generate a UserService class
3// that implements IUserService. It depends on IUserRepository
4// through constructor injection. Use @injectable() and
5// @inject() decorators from tsyringe. Add error handling.
6
7import { injectable, inject } from 'tsyringe';
8import type { IUserService, IUserRepository } from '@/types/interfaces';
9
10@injectable()
11export class UserService implements IUserService {
12  constructor(
13    @inject('IUserRepository')
14    private userRepository: IUserRepository
15  ) {}
16
17  async getUser(id: string): Promise<User> {
18    const user = await this.userRepository.findById(id);
19    if (!user) throw new Error(`User not found: ${id}`);
20    return user;
21  }
22
23  async listUsers(page: number, limit: number): Promise<User[]> {
24    return this.userRepository.findAll(page, limit);
25  }
26
27  async createUser(data: CreateUserDto): Promise<User> {
28    return this.userRepository.create(data);
29  }
30
31  async updateUser(id: string, data: UpdateUserDto): Promise<User> {
32    return this.userRepository.update(id, data);
33  }
34}

1// Cursor Chat prompt (Cmd+L):
2// @src/types/interfaces.ts @src/services/UserService.ts
3// @src/repositories/UserRepository.ts
4// Generate a tsyringe container configuration at
5// src/container.ts that registers:
6// IUserRepository -> UserRepository
7// IUserService -> UserService
8
9import 'reflect-metadata';
10import { container } from 'tsyringe';
11import { UserRepository } from './repositories/UserRepository';
12import { UserService } from './services/UserService';
13
14container.register('IUserRepository', { useClass: UserRepository });
15container.register('IUserService', { useClass: UserService });
16
17export { container };

1// Cursor Chat prompt (Cmd+L):
2// @src/services/UserService.ts @src/types/interfaces.ts
3// Generate a unit test for UserService using Vitest.
4// Mock IUserRepository with vi.fn() implementations.
5// Test getUser with found and not-found scenarios.
6// Do NOT import the real UserRepository.
7
8import { describe, it, expect, vi } from 'vitest';
9import { UserService } from './UserService';
10import type { IUserRepository } from '@/types/interfaces';
11
12const mockRepo: IUserRepository = {
13  findById: vi.fn(),
14  findAll: vi.fn(),
15  create: vi.fn(),
16  update: vi.fn(),
17  delete: vi.fn(),
18};
19
20describe('UserService', () => {
21  const service = new UserService(mockRepo);
22
23  it('returns user when found', async () => {
24    vi.mocked(mockRepo.findById).mockResolvedValue({ id: '1', name: 'Test' });
25    const user = await service.getUser('1');
26    expect(user.name).toBe('Test');
27  });
28
29  it('throws when user not found', async () => {
30    vi.mocked(mockRepo.findById).mockResolvedValue(null);
31    await expect(service.getUser('999')).rejects.toThrow('User not found');
32  });
33});