Provider

Many of the basic Nest classes, such as services, repositories, factories, and helpers, can be treated as providers. The key idea behind a provider is that it can be injected as a dependency, allowing objects to form various relationships with each other.

// interfaces/cat.interface.ts
export interface Cat {
  name: string;
  age: number;
  breed: string;
}

// cats.service.ts
import { Injectable } from "@nestjs/common";
import { Cat } from "./interfaces/cat.interface";

@Injectable()
export class CatsService {
  private readonly cats: Cat[] = [];

  create(cat: Cat) {
    this.cats.push(cat);
  }

  findAll(): Cat[] {
    return this.cats;
  }
}

// cats.controller.ts
import { Controller, Get, Post, Body } from "@nestjs/common";
import { CreateCatDto } from "./dto/create-cat.dto";
import { CatsService } from "./cats.service";
import { Cat } from "./interfaces/cat.interface";

@Controller("cats")
export class CatsController {
  constructor(private catsService: CatsService /* inject the service */) {}

  @Post()
  async create(@Body() createCatDto: CreateCatDto) {
    this.catsService.create(createCatDto);
  }

  @Get()
  async findAll(): Promise<Cat[]> {
    return this.catsService.findAll();
  }
}

// app.module.ts
import { Module } from "@nestjs/common";
import { CatsController } from "./cats/cats.controller";
import { CatsService } from "./cats/cats.service";

@Module({
  controllers: [CatsController],
  providers: [CatsService],
})
export class AppModule {}