NestJSとは – バックエンド用フレームワークでチャットAPIを実装してみる

const app = await NestFactory.create<NestFastifyApplication>(
  AppModule,
  new FastifyAdapter(),
);

$ npx @nestjs/cli new my-project

✨  We will scaffold your app in a few seconds..

✔ Which package manager would you ❤️  to use? pnpm
CREATE my-project/.prettierrc (51 bytes)
CREATE my-project/README.md (5036 bytes)
CREATE my-project/eslint.config.mjs (836 bytes)
CREATE my-project/nest-cli.json (171 bytes)
CREATE my-project/package.json (1988 bytes)
CREATE my-project/tsconfig.build.json (97 bytes)
CREATE my-project/tsconfig.json (677 bytes)
CREATE my-project/src/app.controller.ts (274 bytes)
CREATE my-project/src/app.module.ts (249 bytes)
CREATE my-project/src/app.service.ts (142 bytes)
CREATE my-project/src/main.ts (228 bytes)
CREATE my-project/src/app.controller.spec.ts (617 bytes)
CREATE my-project/test/jest-e2e.json (183 bytes)
CREATE my-project/test/app.e2e-spec.ts (674 bytes)

✔ Installation in progress... ☕

🚀  Successfully created project my-project
👉  Get started with the following commands:

$ cd my-project
$ pnpm run start

> my-project@0.0.1 start /User/xxx/dev/my-project
> nest start

[Nest] 49220  - 09/07/2025, 4:47:56 PM     LOG [NestFactory] Starting Nest application...
[Nest] 49220  - 09/07/2025, 4:47:56 PM     LOG [InstanceLoader] AppModule dependencies initialized +6ms
[Nest] 49220  - 09/07/2025, 4:47:56 PM     LOG [RoutesResolver] AppController {/}: +1ms
[Nest] 49220  - 09/07/2025, 4:47:56 PM     LOG [RouterExplorer] Mapped {/, GET} route +1ms
[Nest] 49220  - 09/07/2025, 4:47:56 PM     LOG [NestApplication] Nest application successfully started +1ms
% curl http://localhost:3000
Hello World!%

./pnpm-lock.yaml
./test
./test/app.e2e-spec.ts
./test/jest-e2e.json
./nest-cli.json
./README.md
./package.json
./tsconfig.build.json
./.prettierrc
./tsconfig.json
./eslint.config.mjs
./src
./src/main.ts
./src/app.service.ts
./src/app.module.ts
./src/app.controller.spec.ts
./src/app.controller.ts

% npx nest g app api

./pnpm-lock.yaml
./nest-cli.json
./README.md
./package.json
./tsconfig.build.json
./.prettierrc
./tsconfig.json
./apps
./apps/my-project
./apps/my-project/tsconfig.app.json
./apps/my-project/test
./apps/my-project/test/app.e2e-spec.ts
./apps/my-project/test/jest-e2e.json
./apps/my-project/src
./apps/my-project/src/main.ts
./apps/my-project/src/app.service.ts
./apps/my-project/src/app.module.ts
./apps/my-project/src/app.controller.spec.ts
./apps/my-project/src/app.controller.ts
./apps/api
./apps/api/tsconfig.app.json
./apps/api/test
./apps/api/test/app.e2e-spec.ts
./apps/api/test/jest-e2e.json
./apps/api/src
./apps/api/src/api.service.ts
./apps/api/src/api.module.ts
./apps/api/src/main.ts
./apps/api/src/api.controller.ts
./apps/api/src/api.controller.spec.ts
./eslint.config.mjs

% npx nest g app worker

% npx nest g lib common
✔ What prefix would you like to use for the library (default: @app or 'defaultLibraryPrefix' setting value)? 
CREATE libs/common/tsconfig.lib.json (220 bytes)
CREATE libs/common/src/index.ts (67 bytes)
CREATE libs/common/src/common.module.ts (192 bytes)
CREATE libs/common/src/common.service.spec.ts (460 bytes)
CREATE libs/common/src/common.service.ts (90 bytes)
UPDATE nest-cli.json (1335 bytes)
UPDATE package.json (2195 bytes)
UPDATE tsconfig.json (817 bytes)

% npx nest g lib database
✔ What prefix would you like to use for the library (default: @app or 'defaultLibraryPrefix' setting value)? 
CREATE libs/database/tsconfig.lib.json (222 bytes)
CREATE libs/database/src/index.ts (71 bytes)
CREATE libs/database/src/database.module.ts (202 bytes)
CREATE libs/database/src/database.service.spec.ts (474 bytes)
CREATE libs/database/src/database.service.ts (92 bytes)
UPDATE nest-cli.json (1578 bytes)
UPDATE package.json (2260 bytes)
UPDATE tsconfig.json (945 bytes)

{
  "$schema": "https://json.schemastore.org/nest-cli",
  "collection": "@nestjs/schematics",
  "sourceRoot": "apps/my-project/src",
  "compilerOptions": {
    "deleteOutDir": true,
    "webpack": true,
    "tsConfigPath": "apps/my-project/tsconfig.app.json"
  },
  "monorepo": true,
  "root": "apps/my-project",
  "projects": {
    "ai-avatar-backend": {
      "type": "application",
      "root": "apps/my-project",
      "entryFile": "main",
      "sourceRoot": "apps/my-project/src",
      "compilerOptions": {
        "tsConfigPath": "apps/my-project/tsconfig.app.json"
      }
    },
    "api": {
      "type": "application",
      "root": "apps/api",
      "entryFile": "main",
      "sourceRoot": "apps/api/src",
      "compilerOptions": {
        "tsConfigPath": "apps/api/tsconfig.app.json"
      }
    },
    "common": {
      "type": "library",
      "root": "libs/common",
      "entryFile": "index",
      "sourceRoot": "libs/common/src",
      "compilerOptions": {
        "tsConfigPath": "libs/common/tsconfig.lib.json"
      }
    },
    "database": {
      "type": "library",
      "root": "libs/database",
      "entryFile": "index",
      "sourceRoot": "libs/database/src",
      "compilerOptions": {
        "tsConfigPath": "libs/database/tsconfig.lib.json"
      }
    },
    "worker": {
      "type": "application",
      "root": "apps/worker",
      "entryFile": "main",
      "sourceRoot": "apps/worker/src",
      "compilerOptions": {
        "tsConfigPath": "apps/worker/tsconfig.app.json"
      }
    }
  }
}

pnpm add openai @nestjs/config

OPENAI_API_KEY=sk-xxxxxx
PORT=3000

import { Module } from '@nestjs/common';
import { ConfigModule } from '@nestjs/config';
import { ChatModule } from './chat/chat.module';

@Module({
  imports: [
    ConfigModule.forRoot({ isGlobal: true }),
    ChatModule,
  ],
})
export class ApiModule {}

import { Module } from '@nestjs/common';
import { ChatService } from './chat.service';
import { ChatController } from './chat.controller';

@Module({
  controllers: [ChatController],
  providers: [ChatService],
})
export class ChatModule {}

import { Injectable } from '@nestjs/common';
import OpenAI from 'openai';

@Injectable()
export class ChatService {
  private client: OpenAI;

  constructor() {
    this.client = new OpenAI({
      apiKey: process.env.OPENAI_API_KEY,
    });
  }

  async getReply(message: string): Promise<string> {
    const completion = await this.client.chat.completions.create({
      model: 'gpt-4o-mini',
      messages: [
        { role: 'system', content: 'You are a helpful care coordinator assistant.' },
        { role: 'user', content: message },
      ],
    });

    return completion.choices[0].message.content ?? '';
  }
}

import { Controller, Post, Body } from '@nestjs/common';
import { ChatService } from './chat.service';

@Controller('chat')
export class ChatController {
  constructor(private readonly chatService: ChatService) {}

  @Post()
  async chat(@Body('message') message: string) {
    if (!message) {
      return { error: 'message is required' };
    }
    const reply = await this.chatService.getReply(message);
    return { reply };
  }
}

% pnpm start api

% curl -X POST http://localhost:3000/chat \
  -H "Content-Type: application/json" \
  -d '{"message":"Hello, can you help me find hair loss products?"}'

{"reply":"Sure! There are various types of hair loss products available, 
including topical treatments, dietary supplements, and devices. 
Here are some popular categories and examples:\n\n1. ... "}