Environment variables in NestJS - Mini Project: Build & Apply

Using Environment Variables in NestJS

📖 Scenario: You are building a simple NestJS application that needs to use environment variables to configure its behavior. Environment variables help keep sensitive data like API keys or configuration settings outside your code.

🎯 Goal: Learn how to set up environment variables in a NestJS project and access them inside a service.

📋 What You'll Learn

Create a .env file with specific variables

Configure the ConfigModule to load environment variables

Inject ConfigService to read environment variables

Use environment variables inside a service method

💡 Why This Matters

🌍 Real World

Environment variables keep sensitive data and configuration outside code, making apps safer and easier to configure for different environments like development and production.

💼 Career

Knowing how to use environment variables is essential for backend developers working with NestJS or any server-side framework to manage configuration securely and flexibly.

1

Create a .env file with variables

Create a file named .env in the root folder with these exact lines:
API_KEY=12345
PORT=3000

NestJS

# Create a .env file with API_KEY=12345 and PORT=3000

Hint

The .env file should be plain text with key=value pairs, no quotes.

2

Configure ConfigModule to load .env

In app.module.ts, import ConfigModule from @nestjs/config and add ConfigModule.forRoot() to the imports array inside @Module.

NestJS

import { Module } from '@nestjs/common';
// Import ConfigModule here

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

Hint

ConfigModule.forRoot() loads environment variables automatically.

3

Inject ConfigService in a service

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

NestJS

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

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

Hint

Use dependency injection to get ConfigService instance.

4

Use environment variables inside a method

Add a method getApiKey() inside AppService that returns the value of the environment variable API_KEY using this.configService.get('API_KEY').

NestJS

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

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

  // Add getApiKey method here
}

Hint

Use ConfigService's get method to read environment variables.

Practice

1. What is the main purpose of using environment variables in a NestJS application?

easy

A. To hardcode sensitive data like passwords in the source code

B. To write all configuration directly inside the main application file

C. To make the application run faster by avoiding configuration files

D. To keep configuration settings outside the code for security and flexibility

Solution

1. Step 1: Understand environment variables purpose

   Environment variables store settings like API keys or database URLs outside the code to keep them safe and easy to change.

2. Step 2: Compare options

   Only To keep configuration settings outside the code for security and flexibility correctly describes this purpose. Options A and B suggest unsafe practices, and D is incorrect about performance.

3. Final Answer:

   To keep configuration settings outside the code for security and flexibility -> Option D

4. Quick Check:

   Environment variables = external safe config [OK]

Hint: Environment variables keep secrets out of code [OK]

Common Mistakes:

• Hardcoding sensitive data inside source code
• Confusing environment variables with regular variables
• Thinking environment variables improve speed

2. Which of the following is the correct way to load environment variables in a NestJS app?

easy

A. Manually read .env file using fs module in every service

B. Use process.env directly without any module

C. Import ConfigModule with ConfigModule.forRoot() in the root module

D. Declare environment variables inside main.ts without ConfigModule

Solution

1. Step 1: Identify NestJS recommended method

   NestJS uses ConfigModule.forRoot() to load .env files and provide ConfigService.

2. Step 2: Evaluate other options

   Options A and C are possible but not recommended patterns. Declare environment variables inside main.ts without ConfigModule is incorrect because environment variables are external.

3. Final Answer:

   Import ConfigModule with ConfigModule.forRoot() in the root module -> Option C

4. Quick Check:

   ConfigModule.forRoot() loads env [OK]

Hint: Use ConfigModule.forRoot() to load .env [OK]

Common Mistakes:

• Reading .env manually everywhere
• Ignoring ConfigModule and using process.env directly
• Declaring env variables inside code files

3. Given this code snippet in a NestJS service:

constructor(private configService: ConfigService) {}

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

If the .env file contains DATABASE_HOST=localhost, what will getDbHost() return?

medium

A. undefined

B. localhost

C. DATABASE_HOST

D. null

Solution

1. Step 1: Understand ConfigService.get() behavior

   ConfigService.get('DATABASE_HOST') reads the value of DATABASE_HOST from loaded environment variables.

2. Step 2: Match with .env content

   The .env file sets DATABASE_HOST=localhost, so the method returns 'localhost'.

3. Final Answer:

   localhost -> Option B

4. Quick Check:

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

Hint: ConfigService.get returns env value by key [OK]

Common Mistakes:

• Expecting the key name instead of value
• Assuming undefined if ConfigModule not imported
• Confusing null with undefined

4. What is wrong with this NestJS module setup for environment variables?

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

medium

A. ConfigModule is imported without calling forRoot(), so env variables won't load

B. ConfigModule should be imported in providers, not imports

C. ConfigModule requires a .json config file, not .env

D. ConfigModule must be imported twice to work

Solution

1. Step 1: Check ConfigModule import syntax

   ConfigModule must be imported with forRoot() to load environment variables from .env file.

2. Step 2: Identify error in code

   The code imports ConfigModule directly without forRoot(), so env variables won't be loaded.

3. Final Answer:

   ConfigModule is imported without calling forRoot(), so env variables won't load -> Option A

4. Quick Check:

   ConfigModule.forRoot() needed to load env [OK]

Hint: Always use ConfigModule.forRoot() to load env [OK]

Common Mistakes:

• Importing ConfigModule without forRoot()
• Placing ConfigModule in providers instead of imports
• Thinking ConfigModule needs JSON config

5. You want to provide a default value for an environment variable PORT in NestJS if it is not set in the .env file. Which code snippet correctly achieves this in a service?

hard

A. const port = this.configService.get('PORT') || 3000;

B. const port = process.env.PORT ? 3000 : this.configService.get('PORT');

C. const port = this.configService.get('PORT', 3000);

D. const port = this.configService.get('PORT') && 3000;

Solution

1. Step 1: Understand default value logic

   If PORT is undefined or empty, use 3000 as default. Logical OR (||) works for this.

2. Step 2: Analyze options

   const port = this.configService.get('PORT') || 3000; uses || to provide default 3000 if get('PORT') is falsy. const port = process.env.PORT ? 3000 : this.configService.get('PORT'); reverses logic incorrectly. const port = this.configService.get('PORT', 3000); is invalid because get() does not take default param. const port = this.configService.get('PORT') && 3000; uses && which returns 3000 only if PORT is truthy, wrong logic.

3. Final Answer:

   const port = this.configService.get('PORT') || 3000; -> Option A

4. Quick Check:

   Use || to set default value [OK]

Hint: Use || to set default if env variable missing [OK]

Common Mistakes:

• Using && instead of || for default
• Passing default value as second argument to get()
• Reversing condition logic