Custom configuration files in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS config service usage?

Given a custom configuration file that exports a function returning an object, what will be the value of configService.get('database.host') after the module loads?

NestJS

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

// config/database.config.ts
export default () => ({
  database: {
    host: 'localhost',
    port: 5432
  }
});

// app.module.ts
import { Module } from '@nestjs/common';
@Module({
  imports: [ConfigModule.forRoot({ load: [require('./config/database.config').default] })],
})
export class AppModule {}

// some.service.ts
import { Injectable } from '@nestjs/common';
import { ConfigService } from '@nestjs/config';

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

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

A 'localhost'

B undefined

C 'database.host'

D null

💡 Hint

Remember that the config loader function returns an object with nested keys.

The ConfigModule.forRoot loads the configuration from the function exported by database.config.ts. The returned object has a database key with a nested host property. So, configService.get('database.host') correctly accesses the nested value 'localhost'.

📝 Syntax

intermediate

Which option correctly imports and uses a custom config file in NestJS?

You want to load a custom config file named app.config.ts that exports a default function returning an object. Which import and usage in ConfigModule.forRoot is correct?

A ConfigModule.forRoot({ load: [import('./config/app.config')] })

B ConfigModule.forRoot({ load: [require('./config/app.config').default] })

C ConfigModule.forRoot({ load: ['./config/app.config'] })

D ConfigModule.forRoot({ load: [() => import('./config/app.config')] })

💡 Hint

The load option expects an array of functions returning config objects.

The load option requires an array of functions that return configuration objects. Using require('./config/app.config').default correctly imports the default exported function. Options B and D use dynamic import incorrectly, and C passes a string instead of a function.

🔧 Debug

advanced

Why does this custom config file cause a runtime error in NestJS?

Consider this custom config file settings.config.ts:

export default () => {
  return {
    apiKey: process.env.API_KEY,
    timeout: parseInt(process.env.TIMEOUT)
  }
}

When running the app, it crashes with TypeError: Cannot read property 'API_KEY' of undefined. What is the cause?

A The config function must return a Promise but returns an object

B parseInt is used incorrectly causing a runtime error

C process.env is undefined because environment variables are not loaded before config runs

D The export default syntax is invalid in TypeScript

💡 Hint

Check if environment variables are available when the config function runs.

The error happens because process.env is undefined at the time the config function runs. This usually means environment variables were not loaded yet. NestJS config module loads environment variables from .env files only if envFilePath is set or ignoreEnvFile is false. Without loading env vars first, process.env is undefined.

❓ state_output

advanced

What is the value of configService.get('feature.enabled') after this config loads?

Given this custom config file feature.config.ts:

export default () => ({
  feature: {
    enabled: process.env.FEATURE_ENABLED === 'true'
  }
});

And the environment variable FEATURE_ENABLED is not set, what does configService.get('feature.enabled') return?

A false

B undefined

C true

D null

💡 Hint

What is the result of comparing undefined to 'true' with ===?

If process.env.FEATURE_ENABLED is undefined, then undefined === 'true' is false. So the enabled property is false.

🧠 Conceptual

expert

Which statement about NestJS custom configuration files is TRUE?

Select the correct statement about how custom configuration files work in NestJS ConfigModule.

A Custom config files must be JSON files; ConfigModule cannot load TypeScript files directly.

B Custom config files can export any value type; ConfigModule uses only the first loaded config file.

C ConfigModule requires custom config files to export classes with static methods for config values.

D Custom config files must export a function returning a plain object; ConfigModule merges all loaded configs into one object.

💡 Hint

Think about how multiple config files combine in NestJS.

In NestJS, custom config files export functions that return plain objects. The ConfigModule merges all loaded config objects into a single configuration object accessible via ConfigService. This allows modular and layered configuration.