Custom Providers

Preview — 3 of 10 questions

What does the useValue provider syntax accomplish?

javascript

const config = { timeout: 5000 };

@Module({
  providers: [{ provide: 'HTTP_CONFIG', useValue: config }],
})
export class HttpModule {}

AIt instantiates a class lazily

BIt creates a new instance per request

CIt runs a factory function on every injection

DIt registers an existing value/object/constant under a token so it can be injected directly

How can you make the token LoggerService resolve to FileLogger in production but ConsoleLogger elsewhere?

javascript

@Module({
  providers: [
    {
      provide: LoggerService,
      useClass: process.env.NODE_ENV === 'production' ? FileLogger : ConsoleLogger,
    },
  ],
})
export class LoggingModule {}

ARegister { provide: LoggerService, useClass: isProd ? FileLogger : ConsoleLogger }

BUse two @Injectable() decorators

CSubclass both at runtime

DIt is impossible to swap implementations

When using useFactory, how do you supply dependencies to the factory function?

javascript

@Module({
  providers: [
    {
      provide: 'DB_CLIENT',
      useFactory: (config: ConfigService) => new DbClient(config.get('DB_URL')),
      inject: [ConfigService],
    },
  ],
})
export class DbModule {}

AImport them as globals inside the factory

BFactories cannot have dependencies

CList their tokens in the inject array; Nest resolves them and passes them as factory arguments in order

DUse @Inject() inside the factory body

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