What I really love about Nest.js is that it's a powerful complete node.js framework that gives you oponionated complete solutions for building backend applications.

It has TypeScript support built in from the start and combines elements of OOP (Object-Oriented Programming), FP (Functional Programming), and FRP (Functional Reactive Programming), as the official website says.

It also has architecture from the get-go, which is amazing for someone like me who likes to work with good structured architecture projects.

The documentation alone is very impressive, and it covers every aspect, including how to set environment variables, but for me
I didn't quite get how to set multiple environments.

Maybe because I'm new to node.js and nestjs, however, when I looked on the internet for articles, they were always showing a way that uses the dotenv library directly instead of using nestjs natively, which uses dotenv under the hood.

And after many hours looking at articles beside the documentation, I worked out a solution that is native to Nestjs and thought I would share it.

First

Let's assume we need 2 environments for our project 'dev' and 'prod'
and we need 2 environment variables per environment: PORT and DATABASE_URL.

1- Create 2 .env files to store  PORT and DATABASE_URL and name the files ( .env.dev and .env.prod.)

PORT=3500

DATABASE_URL="file:./dev.db"`

2- Create an env variable called NODE_ENV, but this time instead of storing it in an .env file, we will inject it in start scripts for dev and prod.

Setting NODE_ENV can be different across platforms (windows, mac, or Linux), so we will install a cross-div package to allow us to set environment variables across platforms.

install using npm

$ npm install --save-dev cross-env

or using pnpm

$ pnpm install --save-dev cross-env

Then, modify your package.json scripts to use cross-env:

"scripts": {
    "start:dev": "cross-env-shell NODE_ENV=dev nest start --watch",
    "start:prod": "cross-env-shell NODE_ENV=prod node dist/main",
}

So now when you run your application with

npm run start:dev 

or

pnpm run start:dev

You have NODE_ENV=dev, and you can use it across the application.

3- In order for Nestjs to read .env files, we have to use ConfigModule and configure it correctly, but first we need to install it.

npm

$ npm i --save @nestjs/config

pnpm

$ pnpm i --save @nestjs/config

4- Use ConfigModule :

@Module({
  imports: [
    ConfigModule.forRoot({
      isGlobal: true,
      envFilePath: `.env.${process.env.NODE_ENV}`,
    }),
  ],
  controllers: [AppController],
  providers: [AppService],
})
export class AppModule {}

envFilePath: is used to specify another path for the .env file. 
or read a different file than the default .env file

.env.${process.env.NODE_ENV} will translate to .env.dev and
If we run the command

npm run start:prod

.env.${process.env.NODE_ENV} would translate to .env.prod.

5- How to access our environment variables
To access env variables, we use ConfigModule.

We can use it in main.ts

async function bootstrap() {
  const app = await NestFactory.create(AppModule);

  const configService = app.get(ConfigService);
  const port = configService.get('PORT');

  await app.listen(port);
}

or if we need it in another place, we use injection.
for example

@Injectable()
export class AppService {
  constructor(private configService: ConfigService) {}

  getHello(): string {
    return (
      this.configService.get('DATABASE_URL')
    );
  }
}