Dependency Injection

import { Module, Injectable, Inject } from "@nestjs/common";
 
@Injectable()
export class UsersRepository {
  findById(id: string) { return { id, name: "Ada" }; }
}
 
@Injectable()
export class UsersService {
  constructor(private readonly repo: UsersRepository) {}
 
  getUser(id: string) {
    return this.repo.findById(id);
  }
}
 
@Module({
  providers: [UsersRepository, UsersService],
  exports: [UsersService],
})
export class UsersModule {}

import { Module, Injectable, Inject } from "@nestjs/common";
import { Test } from "@nestjs/testing";
 
// Interface token for swappable implementations
export const EMAIL_SENDER = Symbol("EMAIL_SENDER");
 
export interface EmailSender {
  send(to: string, subject: string): Promise<void>;
}
 
@Injectable()
export class SmtpEmailSender implements EmailSender {
  async send(to: string, subject: string) {
    console.log(`Sending to ${to}: ${subject}`);
  }
}
 
@Injectable()
export class UsersService {
  constructor(
    @Inject(EMAIL_SENDER) private readonly email: EmailSender,
  ) {}
 
  async register(email: string) {
    await this.email.send(email, "Welcome!");
    return { email };
  }
}
 
@Module({
  providers: [
    UsersService,
    { provide: EMAIL_SENDER, useClass: SmtpEmailSender },
  ],
})
export class UsersModule {}
 
// Test override
const testModule = await Test.createTestingModule({
  imports: [UsersModule],
})
  .overrideProvider(EMAIL_SENDER)
  .useValue({ send: async () => {} })
  .compile();

@Injectable({ scope: Scope.REQUEST })
export class RequestContextService {
  userId?: string;
}