260223:1415 20260223 nextJS & nestJS Best pratices

.agents/skills/nestjs-best-practices/rules/di-avoid-service-locator.md

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+---
+title: Avoid Service Locator Anti-Pattern
+impact: HIGH
+impactDescription: Hides dependencies and breaks testability
+tags: dependency-injection, anti-patterns, testing
+---
+
+## Avoid Service Locator Anti-Pattern
+
+Avoid using `ModuleRef.get()` or global containers to resolve dependencies at runtime. This hides dependencies, makes code harder to test, and breaks the benefits of dependency injection. Use constructor injection instead.
+
+**Incorrect (service locator anti-pattern):**
+
+```typescript
+// Use ModuleRef to get dependencies dynamically
+@Injectable()
+export class OrdersService {
+  constructor(private moduleRef: ModuleRef) {}
+
+  async createOrder(dto: CreateOrderDto): Promise<Order> {
+    // Dependencies are hidden - not visible in constructor
+    const usersService = this.moduleRef.get(UsersService);
+    const inventoryService = this.moduleRef.get(InventoryService);
+    const paymentService = this.moduleRef.get(PaymentService);
+
+    const user = await usersService.findOne(dto.userId);
+    // ... rest of logic
+  }
+}
+
+// Global singleton container
+class ServiceContainer {
+  private static instance: ServiceContainer;
+  private services = new Map<string, any>();
+
+  static getInstance(): ServiceContainer {
+    if (!this.instance) {
+      this.instance = new ServiceContainer();
+    }
+    return this.instance;
+  }
+
+  get<T>(key: string): T {
+    return this.services.get(key);
+  }
+}
+```
+
+**Correct (constructor injection with explicit dependencies):**
+
+```typescript
+// Use constructor injection - dependencies are explicit
+@Injectable()
+export class OrdersService {
+  constructor(
+    private usersService: UsersService,
+    private inventoryService: InventoryService,
+    private paymentService: PaymentService,
+  ) {}
+
+  async createOrder(dto: CreateOrderDto): Promise<Order> {
+    const user = await this.usersService.findOne(dto.userId);
+    const inventory = await this.inventoryService.check(dto.items);
+    // Dependencies are clear and testable
+  }
+}
+
+// Easy to test with mocks
+describe('OrdersService', () => {
+  let service: OrdersService;
+
+  beforeEach(async () => {
+    const module = await Test.createTestingModule({
+      providers: [
+        OrdersService,
+        { provide: UsersService, useValue: mockUsersService },
+        { provide: InventoryService, useValue: mockInventoryService },
+        { provide: PaymentService, useValue: mockPaymentService },
+      ],
+    }).compile();
+
+    service = module.get(OrdersService);
+  });
+});
+
+// VALID: Factory pattern for dynamic instantiation
+@Injectable()
+export class HandlerFactory {
+  constructor(private moduleRef: ModuleRef) {}
+
+  getHandler(type: string): Handler {
+    switch (type) {
+      case 'email':
+        return this.moduleRef.get(EmailHandler);
+      case 'sms':
+        return this.moduleRef.get(SmsHandler);
+      default:
+        return this.moduleRef.get(DefaultHandler);
+    }
+  }
+}
+```
+
+Reference: [NestJS Module Reference](https://docs.nestjs.com/fundamentals/module-ref)