ConfigService | Engineering Interview Kit

The ConfigService is the injectable provider that gives you access to the parsed environment variables throughout your application.

Overview

Once the ConfigModule is registered, the ConfigService becomes available via Dependency Injection. You inject it into your controllers, services, or even other modules to retrieve configuration values.

It is superior to process.env because it provides type casting (e.g., converting a string “3000” into a number 3000), allows for default fallback values, and can throw errors if a required variable is missing.

Key Concepts

Type Hinting: configService.get<T>('KEY') allows you to tell TypeScript what type you expect the variable to be.
Default Values: configService.get('KEY', 'default_value') provides a fallback if the key is missing in the .env file.
infer: true: A powerful TypeScript feature that infers nested configuration types.

Code Examples

1. Basic Retrieval and Type Casting

Notice how we can cast the PORT from a string to a number.

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

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

  sendEmail() {
    // 1. Get a string
    const apiKey = this.configService.get<string>('SENDGRID_API_KEY');

    // 2. Get a number with a default fallback if the variable doesn't exist
    const maxRetries = this.configService.get<number>('EMAIL_RETRIES', 3);

    // 3. Get a boolean (e.g., if LOG_EMAILS=true)
    const shouldLog = this.configService.get<boolean>('LOG_EMAILS');
  }
}

2. Async Configuration in Modules (The `forRootAsync` Pattern)

You often need environment variables before a service is instantiated—specifically when configuring third-party modules like databases or cache servers in your AppModule.

You cannot just use process.env here (it defeats the purpose of the ConfigModule). Instead, you use the useFactory pattern to inject the ConfigService into the module’s setup phase.

import { Module } from '@nestjs/common';
import { TypeOrmModule } from '@nestjs/typeorm';
import { ConfigModule, ConfigService } from '@nestjs/config';

@Module({
  imports: [
    ConfigModule.forRoot({ isGlobal: true }),
    
    // Using forRootAsync instead of forRoot!
    TypeOrmModule.forRootAsync({
      imports: [ConfigModule], // Tell TypeORM to import the ConfigModule
      inject: [ConfigService], // Inject the ConfigService into the factory
      
      // The factory receives the ConfigService and returns the configuration object
      useFactory: (configService: ConfigService) => ({
        type: 'postgres',
        host: configService.get<string>('DATABASE_HOST'),
        port: configService.get<number>('DATABASE_PORT'),
        username: configService.get<string>('DATABASE_USER'),
        password: configService.get<string>('DATABASE_PASS'),
      }),
    }),
  ],
})
export class AppModule {}

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