Name: oh-my-ai/nestjs
Rating: 99.7 (1 reviews)
Author: oh-my-ai

oh-my-ai/nestjs

NestJS architecture, dependency injection, validation, security, errors, testing, persistence, APIs, microservices, and deployment patterns with prioritized rule tiers and companion rule files.

1.12x

Quality

100%

Does it follow best practices?

Impact

97%

1.12x

Average score across 2 eval scenarios

Securityby

Advisory

Suggest reviewing before use

name:: di-use-interfaces-tokens
description:: Use Injection Tokens for Interfaces — Enables interface-based DI at runtime
title:: Use Injection Tokens for Interfaces
impact:: HIGH
impactDescription:: Enables interface-based DI at runtime
tags:: dependency-injection, tokens, interfaces

Use Injection Tokens for Interfaces

TypeScript interfaces are erased at compile time and can't be used as injection tokens. Use string tokens, symbols, or abstract classes when you want to inject implementations of interfaces. This enables swapping implementations for testing or different environments.

Incorrect (interface can't be used as token):

// Interface can't be used as injection token
interface PaymentGateway {
  charge(amount: number): Promise<PaymentResult>;
}

@Injectable()
export class StripeService implements PaymentGateway {
  charge(amount: number) { /* ... */ }
}

@Injectable()
export class OrdersService {
  // This WON'T work - PaymentGateway doesn't exist at runtime
  constructor(private payment: PaymentGateway) {}
}

Correct (symbol tokens or abstract classes):

// Option 1: String/Symbol tokens (most flexible)
export const PAYMENT_GATEWAY = Symbol('PAYMENT_GATEWAY');

export interface PaymentGateway {
  charge(amount: number): Promise<PaymentResult>;
}

@Injectable()
export class StripeService implements PaymentGateway {
  async charge(amount: number): Promise<PaymentResult> {
    // Stripe implementation
  }
}

@Injectable()
export class MockPaymentService implements PaymentGateway {
  async charge(amount: number): Promise<PaymentResult> {
    return { success: true, id: 'mock-id' };
  }
}

// Module registration
@Module({
  providers: [
    {
      provide: PAYMENT_GATEWAY,
      useClass: process.env.NODE_ENV === 'test'
        ? MockPaymentService
        : StripeService,
    },
  ],
  exports: [PAYMENT_GATEWAY],
})
export class PaymentModule {}

// Injection
@Injectable()
export class OrdersService {
  constructor(
    @Inject(PAYMENT_GATEWAY) private payment: PaymentGateway,
  ) {}

  async createOrder(dto: CreateOrderDto) {
    await this.payment.charge(dto.amount);
  }
}

// Option 2: Abstract class (carries runtime type info)
export abstract class PaymentGateway {
  abstract charge(amount: number): Promise<PaymentResult>;
}

@Injectable()
export class StripeService extends PaymentGateway {
  async charge(amount: number): Promise<PaymentResult> {
    // Implementation
  }
}

// No @Inject needed with abstract class
@Injectable()
export class OrdersService {
  constructor(private payment: PaymentGateway) {}
}

Reference: NestJS Custom Providers