Nest.js config loading utility

Yet another way of loading config in Nest.js.

Description

This library is heavily inspired by solution described in Nest.js docs.

Basically it does three things:

• reads config from environment variables just like Nest's ConfigModule would;
• with help from class-transformer creates strongly-typed, injectable class instances with properties mapped from environment variables;
• using the power of class-validator validates these variables.

Usage

Create config classes

Create regular TypeScript classes and mark the properties you want mapped from environment variables.

export enum Environment {
  Development = "development",
  Production = "production"
}

export class ServerConfig {

  // Providing environment variable name explicitly
  @IsEnum(Environment)
  @EnvProperty('NODE_ENV')
  readonly environment: Environment;

  // `SERVER_PORT` will be used as an environment variable name
  @IsNumber()
  @EnvProperty()
  readonly SERVER_PORT: number;
}

export class DatabaseConfig {

  @Matches(/^(mongodb:(?:\/{2})?)((\w+?):(\w+?)@|:?@?)(\w+?):(\d+)\/([\w-]+?)$/)
  @EnvProperty('APP_MONGO_URI')
  readonly mongoUri: string;
  
}

:warning: You have to use at least one class-validator decorator per class.

Import ConfigModule

In AppModule:

@Module({
  imports: [
    ConfigModule.forRoot({
      classes: [ServerConfig, DatabaseConfig]
    })
  ],
  controllers: [],
  providers: [],
})
export class AppModule {}

You can customize behavior of almost any initialization step performed by external packages, as well as provide custom error to throw when validation fails. Check ConfigModuleOptions for more info.

Use your classes in the app

Your instantiated config classes are available via good old dependency injection:

@Injectable()
export class DatabaseConfigService {

  constructor(
    config: DatabaseConfig
  ) {
    console.log(config.mongoUri); // => "mongodb://localhost:27017/my-database"
  }

}

Original config object resolved by Nest is passed through the module instantiation process, so all environment variables will be available via ConfigService#get, though without typing:

@Injectable()
export class ServerConfigService {

  constructor(
    configService: ConfigService
  ) {
    const env = configService.get('NODE_ENV');      // type = any
    console.log(env);                               // => development
  }

}

Use in main.ts

Since initialization of the module happens at the decorators level, your config classes are available even before creation of the app in main.ts!

async function bootstrap() {
  const config = ConfigResolver.get(AppConfig)!;
  
  // Use your config even during `AppModule` creation.
  const app = await NestFactory.create(AppModule);
  await app.listen(config.serverPort);
}
bootstrap();