Environment-based configuration in NestJS - Mini Project: Build & Apply

Environment-based configuration in NestJS

📖 Scenario: You are building a simple NestJS application that needs to read configuration values from environment variables. This is common when you want to change settings like the app port or database URL without changing the code.

🎯 Goal: Learn how to set up environment-based configuration in NestJS using the ConfigModule. You will create a configuration file, load environment variables, and access them in a service.

📋 What You'll Learn

Create a configuration file that exports a function returning an object with app settings

Import and configure the ConfigModule to load environment variables

Inject ConfigService into a service to read configuration values

Use the configuration values in the service logic

💡 Why This Matters

🌍 Real World

Many real-world NestJS apps need to change settings like ports, database URLs, or API keys without changing code. Environment-based configuration makes this easy and safe.

💼 Career

Understanding environment-based configuration is essential for backend developers working with NestJS, as it helps build flexible and secure applications that adapt to different deployment environments.

1

Create a configuration file

Create a file named app.config.ts that exports a default function returning an object with these exact properties: port set to 3000 and databaseUrl set to "mongodb://localhost:27017/myapp".

NestJS

// Create and export default function returning { port: 3000, databaseUrl: 'mongodb://localhost:27017/myapp' }
// Your code here

Hint

This file returns an object with your app settings. Use export default () => ({ ... }).

2

Import ConfigModule with your config

In app.module.ts, import ConfigModule from @nestjs/config and configure it with ConfigModule.forRoot({ load: [appConfig] }). Also, import your app.config.ts as appConfig.

NestJS

import { Module } from '@nestjs/common';
import { ConfigModule } from '@nestjs/config';
import appConfig from './app.config';

@Module({
  imports: [
    // Configure ConfigModule here
  ],
})
export class AppModule {}

// Your code here

Hint

Use ConfigModule.forRoot with the load option to load your config function.

3

Inject ConfigService in a service

Create a service named AppService that injects ConfigService from @nestjs/config in its constructor. Store it in a private readonly property named configService.

NestJS

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

@Injectable()
export class AppService {
  constructor(
    // Inject ConfigService here
  ) {}
}

// Your code here

Hint

Use constructor injection with private readonly configService: ConfigService.

4

Use configuration values in the service

Add a method getPort() in AppService that returns the port value from configService.get<number>('port'). Also add a method getDatabaseUrl() that returns the databaseUrl string from configService.get<string>('databaseUrl').

NestJS

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

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

  // Add getPort() and getDatabaseUrl() methods here

  // Your code here
}

Hint

Use configService.get<type>('key') to read config values.

Practice

1. What is the main purpose of using environment-based configuration in a NestJS application?

easy

A. To write more complex code logic

B. To hardcode all secrets inside the source files

C. To increase the size of the application

D. To change app settings without modifying the code

Solution

1. Step 1: Understand environment-based configuration

   It allows changing settings like database URLs or API keys without changing the app code.

2. Step 2: Identify the correct purpose

   Changing settings without code changes helps adapt the app to different environments easily.

3. Final Answer:

   To change app settings without modifying the code -> Option D

4. Quick Check:

   Environment config = change settings without code [OK]

Hint: Think: config changes without touching code [OK]

Common Mistakes:

• Confusing config with code logic
• Assuming config increases app size
• Hardcoding secrets inside source files

2. Which NestJS module is used to load environment variables for configuration?

easy

A. ConfigModule

B. DatabaseModule

C. HttpModule

D. LoggerModule

Solution

1. Step 1: Recall NestJS modules for config

   The ConfigModule is designed to load and manage environment variables.

2. Step 2: Match module to purpose

   HttpModule handles HTTP, DatabaseModule handles DB, LoggerModule handles logs, so ConfigModule is correct.

3. Final Answer:

   ConfigModule -> Option A

4. Quick Check:

   ConfigModule loads env variables [OK]

Hint: ConfigModule = environment variables loader [OK]

Common Mistakes:

• Choosing unrelated modules like HttpModule
• Confusing ConfigModule with DatabaseModule
• Assuming LoggerModule manages config

3. Given this code snippet in a NestJS service:

constructor(private configService: ConfigService) {}

getDbHost() {
  return this.configService.get('DATABASE_HOST');
}

What will getDbHost() return if the environment variable DATABASE_HOST is set to localhost?

medium

A. DATABASE_HOST

B. undefined

C. localhost

D. null

Solution

1. Step 1: Understand ConfigService.get()

   This method returns the value of the environment variable key passed, here 'DATABASE_HOST'.

2. Step 2: Check environment variable value

   The variable DATABASE_HOST is set to 'localhost', so get() returns 'localhost'.

3. Final Answer:

   localhost -> Option C

4. Quick Check:

   ConfigService.get('DATABASE_HOST') = 'localhost' [OK]

Hint: ConfigService.get returns env var value [OK]

Common Mistakes:

• Expecting the key name instead of its value
• Assuming undefined if variable exists
• Confusing null with undefined

4. You wrote this code to load environment variables in NestJS:

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

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

But your app crashes with an error saying environment variables are undefined. What is the likely cause?

medium

A. You must call ConfigService.get() inside main.ts only

B. You forgot to create a .env file or set environment variables

C. You need to import HttpModule as well

D. ConfigModule.forRoot() should be called inside a service, not module

Solution

1. Step 1: Check environment variable source

   ConfigModule loads variables from .env file or system environment. If missing, variables are undefined.

2. Step 2: Identify common mistake

   Not having a .env file or not setting variables causes undefined values and app crash.

3. Final Answer:

   You forgot to create a .env file or set environment variables -> Option B

4. Quick Check:

   Missing .env file = undefined env vars [OK]

Hint: Always create .env file or set env vars [OK]

Common Mistakes:

• Thinking ConfigModule.forRoot() is misplaced
• Assuming HttpModule is needed for config
• Calling ConfigService.get() only in main.ts

5. You want to load different database URLs for development and production in NestJS using environment variables. Which approach correctly uses ConfigModule to load .env.development or .env.production based on NODE_ENV?

hard

A. ConfigModule.forRoot({ envFilePath: `.env.${process.env.NODE_ENV}` })

B. ConfigModule.forRoot({ envFilePath: '.env' })

C. ConfigModule.forRoot({ envFilePath: '.env.production' }) always

D. ConfigModule.forRoot({ envFilePath: '.env.development' }) always

Solution

1. Step 1: Understand dynamic env file loading

   Using envFilePath with template string allows loading .env files based on NODE_ENV value.

2. Step 2: Check options

   ConfigModule.forRoot({ envFilePath: `.env.${process.env.NODE_ENV}` }) dynamically loads .env.development or .env.production depending on NODE_ENV, others load fixed files.

3. Final Answer:

   ConfigModule.forRoot({ envFilePath: `.env.${process.env.NODE_ENV}` }) -> Option A

4. Quick Check:

   Dynamic envFilePath = correct multi-env config [OK]

Hint: Use template string with NODE_ENV for envFilePath [OK]

Common Mistakes:

• Hardcoding envFilePath to one file always
• Not using process.env.NODE_ENV in path
• Assuming default .env handles all environments