Configuration and custom providers

Not everything is a class

Config objects, API clients and feature flags aren't services you write — but you still want them injected. Custom providers register *any* value under an injection token, and @Inject(token) asks for it.

Three kinds of custom provider

import { Controller, Get, Inject, Injectable, Module } from "@nestjs/common";
import { NestFactory } from "@nestjs/core";

export const CONFIG = "CONFIG";

@Injectable()
class PricingService {
  constructor(@Inject(CONFIG) private readonly config: { taxRate: number; currency: string }) {}
  withTax(price: number) {
    return `${this.config.currency} ${Math.round(price * (1 + this.config.taxRate))}`;
  }
}

@Controller()
class PricesController {
  constructor(private readonly pricing: PricingService, @Inject("OPENED_AT") private readonly openedAt: string) {}
  @Get("momo")
  momo() {
    return { price: this.pricing.withTax(180), openedAt: this.openedAt };
  }
}

@Module({
  controllers: [PricesController],
  providers: [
    PricingService,
    { provide: CONFIG, useValue: { taxRate: 0.13, currency: "Rs" } },
    { provide: "OPENED_AT", useFactory: (cfg: { currency: string }) => `opened with ${cfg.currency}`, inject: [CONFIG] },
  ],
})
class AppModule {}

const app = await NestFactory.create(AppModule);
await app.listen(3000);
console.log(await (await fetch("http://localhost:3000/momo")).json());

useClass swaps an implementation: { provide: PaymentsService, useClass: FakePaymentsService } in development.
Real apps use @nestjs/config (ConfigModule.forRoot(), ConfigService.get("PORT")) — built on exactly these providers.

Not everything is a class

Three kinds of custom provider

The rest of this lesson