1---
2title: Use Injection Tokens for Interfaces
3impact: HIGH
4impactDescription: Enables interface-based DI at runtime
5tags: dependency-injection, tokens, interfaces
6---
7 
8## Use Injection Tokens for Interfaces
9 
10TypeScript 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.
11 
12**Incorrect (interface can't be used as token):**
13 
14```typescript
15// Interface can't be used as injection token
16interface PaymentGateway {
17  charge(amount: number): Promise<PaymentResult>;
18}
19 
20@Injectable()
21export class StripeService implements PaymentGateway {
22  charge(amount: number) { /* ... */ }
23}
24 
25@Injectable()
26export class OrdersService {
27  // This WON'T work - PaymentGateway doesn't exist at runtime
28  constructor(private payment: PaymentGateway) {}
29}
30```
31 
32**Correct (symbol tokens or abstract classes):**
33 
34```typescript
35// Option 1: String/Symbol tokens (most flexible)
36export const PAYMENT_GATEWAY = Symbol('PAYMENT_GATEWAY');
37 
38export interface PaymentGateway {
39  charge(amount: number): Promise<PaymentResult>;
40}
41 
42@Injectable()
43export class StripeService implements PaymentGateway {
44  async charge(amount: number): Promise<PaymentResult> {
45    // Stripe implementation
46  }
47}
48 
49@Injectable()
50export class MockPaymentService implements PaymentGateway {
51  async charge(amount: number): Promise<PaymentResult> {
52    return { success: true, id: 'mock-id' };
53  }
54}
55 
56// Module registration
57@Module({
58  providers: [
59    {
60      provide: PAYMENT_GATEWAY,
61      useClass: process.env.NODE_ENV === 'test'
62        ? MockPaymentService
63        : StripeService,
64    },
65  ],
66  exports: [PAYMENT_GATEWAY],
67})
68export class PaymentModule {}
69 
70// Injection
71@Injectable()
72export class OrdersService {
73  constructor(
74    @Inject(PAYMENT_GATEWAY) private payment: PaymentGateway,
75  ) {}
76 
77  async createOrder(dto: CreateOrderDto) {
78    await this.payment.charge(dto.amount);
79  }
80}
81 
82// Option 2: Abstract class (carries runtime type info)
83export abstract class PaymentGateway {
84  abstract charge(amount: number): Promise<PaymentResult>;
85}
86 
87@Injectable()
88export class StripeService extends PaymentGateway {
89  async charge(amount: number): Promise<PaymentResult> {
90    // Implementation
91  }
92}
93 
94// No @Inject needed with abstract class
95@Injectable()
96export class OrdersService {
97  constructor(private payment: PaymentGateway) {}
98}
99```
100 
101Reference: [NestJS Custom Providers](https://docs.nestjs.com/fundamentals/custom-providers)
102