1. Introducing the Config Module

The @nestjs/config helps manage environment variables (.env) in a clean and scalable way.

👉 Why use it?

• Centralized config
• Avoid hardcoding values
• Environment-based setups (dev, prod, test)

Example:

import { ConfigModule } from '@nestjs/config';

@Module({
  imports: [ConfigModule.forRoot()],
})
export class AppModule {}

Now you can use .env:

PORT=3000
DB_HOST=localhost

2. Custom Environment File Paths

You can load different .env files depending on environment.

👉 Useful for:

• .env.development
• .env.production

Example:

ConfigModule.forRoot({
  envFilePath: '.env.development',
});

Or multiple:

ConfigModule.forRoot({
  envFilePath: ['.env.development', '.env'],
});

3. Schema Validation

Use Joi to validate environment variables at startup.

👉 Prevents runtime errors due to missing/invalid config.

Example:

import * as Joi from 'joi';

ConfigModule.forRoot({
  validationSchema: Joi.object({
    PORT: Joi.number().required(),
    DB_HOST: Joi.string().required(),
  }),
});

4. Using the Config Service

Use ConfigService to access config values anywhere.

Example:

import { ConfigService } from '@nestjs/config';

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

  getPort() {
    return this.configService.get<number>('PORT');
  }
}

👉 Pro tip: Add default values

this.configService.get<number>('PORT', 3000);

5. Custom Configuration Files

Instead of using only .env, you can define structured config files.

👉 Good for grouping related config (DB, auth, etc.)

Example:

// config/database.config.ts
export default () => ({
  database: {
    host: process.env.DB_HOST,
    port: process.env.DB_PORT,
  },
});

Register:

ConfigModule.forRoot({
  load: [databaseConfig],
});

Use:

configService.get('database.host');

6. Configuration Namespaces and Partial Registration

Use registerAs to create namespaced configs.

👉 Helps avoid key collisions and improves structure.

Example:

import { registerAs } from '@nestjs/config';

export default registerAs('database', () => ({
  host: process.env.DB_HOST,
  port: process.env.DB_PORT,
}));

Register:

ConfigModule.forRoot({
  load: [databaseConfig],
});

Use:

configService.get('database.host');

👉 Partial registration (only in specific module):

ConfigModule.forFeature(databaseConfig);

7. Asynchronously Configure Dynamic Modules

Some modules need async config (e.g., DB connection).

👉 Use forRootAsync when config depends on ConfigService.

Example (TypeORM):

TypeOrmModule.forRootAsync({
  imports: [ConfigModule],
  inject: [ConfigService],
  useFactory: (config: ConfigService) => ({
    type: 'postgres',
    host: config.get('DB_HOST'),
    port: config.get('DB_PORT'),
  }),
});

👉 Why this matters:

• Load config dynamically
• Supports secrets, external services
• Cleaner dependency injection

🎯 Quick Summary (for interview)

• ConfigModule → manage .env
• envFilePath → support multiple environments
• Joi validation → fail fast on invalid config
• ConfigService → access values globally
• Custom config files → structured config
• Namespaces → avoid conflicts, better organization
• forRootAsync → dynamic + async configuration

If you want, I can turn this into a blog-style article (like your previous DI blog) or give you real-world project structure (enterprise-level) — that’s usually what interviewers care about most.