Applications often run in different environments. Depending on the environment, different configuration settings should be used. For example, usually the local environment relies on specific database credentials, valid only for the local DB instance. The production environment would use a separate set of DB credentials. Since configuration variables change, best practice is to store configuration variables in the environment.
Externally defined environment variables are visible inside Node.js through the process.env global. We could try to solve the problem of multiple environments by setting the environment variables separately in each environment. This can quickly get unwieldy, especially in the development and testing environments where these values need to be easily mocked and/or changed.
In Node.js applications, it's common to use .env files, holding key-value pairs where each key represents a particular value, to represent each environment. Running an app in different environments is then just a matter of swapping in the correct .env file.
A good approach for using this technique in Nest is to create a ConfigModule that exposes a ConfigService which loads the appropriate .env file. While you may choose to write such a module yourself, for convenience Nest provides the @nestjs/config package out-of-the box.
$ npm i --save @nestjs/config dotenv
Once the installation process is complete, we can import the ConfigModule. Typically, we'll import it into the root AppModule and control its behavior using the .forRoot() static method. During this step, environment variable key/value pairs are parsed and resolved. Later, we'll see several options for accessing the ConfigService class of the ConfigModule in our other feature modules.
import { Module } from '@nestjs/common';
import { ConfigModule } from '@nestjs/config';
@Module({
imports: [ConfigModule.forRoot()],
})
export class AppModule {}
The above code will load and parse a .env file from the default location (the project root directory), merge key/value pairs from the .env file with environment variables assigned to process.env, and store the result in a private structure that you can access through the ConfigService. The forRoot() method registers the ConfigService provider, which provides a get() method for reading these parsed/merged configuration variables. Since @nestjs/config relies on dotenv, it uses that package's rules for resolving conflicts in environment variable names. When a key exists both in the runtime environment as an environment variable (e.g., via OS shell exports like export DATABASE_USER=test) and in a .env file, the runtime environment variable takes precedence.
topic : understanding configuration in nest-way
read this
reference