2 · Data and boundaries · 30 MIN

Nest modules, controllers and providers

Controllers translate HTTP while providers own reusable behavior.

Nest uses modules to register controllers and providers in a dependency-injection graph. A controller receives its service through the constructor instead of constructing it for every request. This separation lets a test replace the service with a controlled implementation. The default provider scope is shared for the application, so never put a particular user’s mutable session state in an ordinary singleton service. The array below is fixed public demonstration data and is not durable storage.

Register and import the module into the application.

Read the example

// courses.service.ts
import {Injectable} from '@nestjs/common';
@Injectable() export class CoursesService {list(){return [{id:'react',title:'React foundations'}]}}
// courses.controller.ts
import {Controller,Get} from '@nestjs/common';
import {CoursesService} from './courses.service';
@Controller('courses') export class CoursesController {
  constructor(private readonly courses:CoursesService){}
  @Get() list(){return this.courses.list()}
}
// courses.module.ts
import {Module} from '@nestjs/common';
import {CoursesController} from './courses.controller';
import {CoursesService} from './courses.service';
@Module({controllers:[CoursesController],providers:[CoursesService]}) export class CoursesModule {}

After importing CoursesModule into AppModule, GET /courses returns the public course array.

Your challenge

Add a find-by-ID service method, return a 404 for unknown IDs and test the controller with a stubbed service.

Solution cost: O(n) teaching-array lookup; list construction is proportional to output. time · O(n) public result data. space

Common trap

Unregistered providers cannot be injected simply because their class exists.

// courses.service.ts
import {Injectable} from '@nestjs/common';
@Injectable() export class CoursesService {list(){return [{id:'react',title:'React foundations'}]}}
// courses.controller.ts
import {Controller,Get} from '@nestjs/common';
import {CoursesService} from './courses.service';
@Controller('courses') export class CoursesController {
  constructor(private readonly courses:CoursesService){}
  @Get() list(){return this.courses.list()}
}
// courses.module.ts
import {Module} from '@nestjs/common';
import {CoursesController} from './courses.controller';
import {CoursesService} from './courses.service';
@Module({controllers:[CoursesController],providers:[CoursesService]}) export class CoursesModule {}

Further reading: Official documentation