Custom configuration files in NestJS - Mini Project: Build & Apply

Custom configuration files in NestJS

📖 Scenario: You are building a NestJS application that needs to load custom configuration settings from a separate file. This helps keep your app settings organized and easy to change without touching the main code.

🎯 Goal: Create a custom configuration file and load it into your NestJS app using the ConfigModule. You will define a config file, import it, and access its values in a service.

📋 What You'll Learn

Create a configuration file named app.config.ts exporting a function returning an object with a port number and database object with host and port.

Import the custom config file into the ConfigModule.forRoot() using the load option.

Inject ConfigService into a service and read the port and database.host values.

Log the loaded configuration values inside the service constructor.

💡 Why This Matters

🌍 Real World

Custom configuration files help keep app settings organized and separate from code. This makes apps easier to maintain and configure for different environments.

💼 Career

Many backend jobs require managing app settings cleanly. Knowing how to use NestJS ConfigModule with custom config files is a valuable skill for building scalable, maintainable applications.

1

Create the custom 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 database as an object with host set to 'localhost' and port set to 5432.

NestJS

// Create and export the config function here
// Your code here

Hint

Use export default () => ({ ... }) to export the config object.

2

Load the custom config in ConfigModule

In your main module file (e.g., app.module.ts), import ConfigModule from @nestjs/config. Then, call ConfigModule.forRoot() with the load option set to an array containing the imported app.config function.

NestJS

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

@Module({
  imports: [
    // Load the custom config here
    // Your code here
  ],
})
export class AppModule {}

Hint

Use ConfigModule.forRoot({ load: [appConfig] }) inside the imports array.

3

Inject ConfigService and read config values

Create a service named AppService. Inject ConfigService from @nestjs/config in its constructor. Inside the constructor, read the port value using this.configService.get('port') and the database.host value using this.configService.get('database.host'). Store these values in class properties named appPort and dbHost respectively.

NestJS

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

@Injectable()
export class AppService {
  appPort: number;
  dbHost: string;

  constructor(private configService: ConfigService) {
    // Read config values here
    // Your code here
  }
}

Hint

Use this.configService.get('port') and this.configService.get('database.host') inside the constructor.

4

Log the configuration values in the service

Inside the AppService constructor, add two console.log statements to print the appPort and dbHost values with these exact messages: App running on port: {appPort} and Database host: {dbHost}.

NestJS

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

@Injectable()
export class AppService {
  appPort: number;
  dbHost: string;

  constructor(private configService: ConfigService) {
    this.appPort = this.configService.get('port');
    this.dbHost = this.configService.get('database.host');
    // Log the config values here
    // Your code here
  }
}

Hint

Use template strings inside console.log to show the values.

Practice

1. What is the main purpose of using custom configuration files in a NestJS application?

easy

A. To write all logic inside configuration files

B. To keep app settings organized and separate from the main code

C. To replace the main app module

D. To store user data permanently

Solution

1. Step 1: Understand configuration role

   Custom configuration files are used to keep settings like API keys or database info separate from the main code.

2. Step 2: Identify the correct purpose

   This separation helps organize the app and makes it easier to change settings without touching code logic.

3. Final Answer:

   To keep app settings organized and separate from the main code -> Option B

4. Quick Check:

   Custom config files organize settings [OK]

Hint: Custom configs separate settings from code for clarity [OK]

Common Mistakes:

• Thinking config files hold app logic
• Confusing config files with database storage
• Assuming config files replace main modules

2. Which syntax correctly creates a named custom configuration file using registerAs in NestJS?

easy

A. export const config = registerAs('app', () => ({ port: 3000 }));

B. registerAs('app', { port: 3000 });

C. export default registerAs('app', () => ({ port: 3000 }));

D. registerAs('app', () => port: 3000);

Solution

1. Step 1: Understand registerAs usage

   The registerAs function takes a name and a function returning a config object.

2. Step 2: Check syntax correctness

   export const config = registerAs('app', () => ({ port: 3000 })); correctly exports a constant assigned to registerAs with a function returning an object.

3. Final Answer:

   export const config = registerAs('app', () => ({ port: 3000 })); -> Option A

4. Quick Check:

   registerAs needs a name and a function returning an object [OK]

Hint: Use export const with registerAs and a function returning an object [OK]

Common Mistakes:

• Omitting export or const keywords
• Passing object directly instead of a function
• Incorrect arrow function syntax

3. Given this custom config file:

export const databaseConfig = registerAs('database', () => ({ host: 'localhost', port: 5432 }));

And this module setup:

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

What will configService.get('database.host') return?

medium

A. Error: Config not found

B. 'database.host'

C. undefined

D. 'localhost'

Solution

1. Step 1: Understand config loading

   The databaseConfig is loaded into ConfigModule with load, making its keys accessible.

2. Step 2: Access nested config value

   Using configService.get('database.host') accesses the 'host' key inside the 'database' config, which is 'localhost'.

3. Final Answer:

   'localhost' -> Option D

4. Quick Check:

   Nested config keys return their values [OK]

Hint: Use dot notation to get nested config values [OK]

Common Mistakes:

• Expecting the whole object instead of a value
• Using wrong key names
• Not loading config before accessing

4. What is wrong with this custom config file code?

export const appConfig = registerAs('app', () => { port: 3000 });

medium

A. The config name 'app' is invalid

B. Missing import of registerAs

C. The arrow function does not return the object properly

D. The port value must be a string

Solution

1. Step 1: Check arrow function return

   The arrow function uses curly braces without a return statement, so it returns undefined instead of the object.

2. Step 2: Correct syntax for returning object

   To return an object directly, wrap it in parentheses: () => ({ port: 3000 }).

3. Final Answer:

   The arrow function does not return the object properly -> Option C

4. Quick Check:

   Arrow functions need parentheses to return objects directly [OK]

Hint: Wrap object in parentheses to return from arrow function [OK]

Common Mistakes:

• Using curly braces without return in arrow functions
• Assuming config name must be special
• Thinking port must be string

5. You want to create two custom config files, appConfig and dbConfig, and load both in your NestJS app. Which setup correctly loads and accesses the dbConfig.username value?

hard

A. Load both configs in ConfigModule.forRoot({ load: [appConfig, dbConfig] }) and get with configService.get('dbConfig.username')

B. Load only appConfig and get dbConfig.username directly

C. Load both configs but access dbConfig.username with configService.get('username')

D. Load configs separately in different modules without forRoot

Solution

1. Step 1: Load multiple configs together

   Use ConfigModule.forRoot({ load: [appConfig, dbConfig] }) to load both custom configs at app start.

2. Step 2: Access nested config by namespace

   Access dbConfig.username using configService.get('dbConfig.username') because configs are namespaced by their registration name.

3. Final Answer:

   Load both configs in ConfigModule.forRoot({ load: [appConfig, dbConfig] }) and get with configService.get('dbConfig.username') -> Option A

4. Quick Check:

   Load all configs and use namespaced keys to access [OK]

Hint: Load all configs in forRoot and use dot keys to access [OK]

Common Mistakes:

• Loading only one config but accessing another
• Accessing keys without namespace prefix
• Trying to load configs outside forRoot