Optional providers in NestJS - Mini Project: Build & Apply

Using Optional Providers in NestJS

📖 Scenario: You are building a NestJS service that optionally uses a logger service if it is provided. This is common when you want to add extra features without forcing them.

🎯 Goal: Create a NestJS service that optionally injects a LoggerService. If the logger is provided, the service uses it to log messages; if not, it works without errors.

📋 What You'll Learn

Create a LoggerService class with a log(message: string) method.

Create a MyService class that optionally injects LoggerService using @Optional().

In MyService, add a method doWork() that calls logger.log() only if the logger exists.

Set up a NestJS module that provides MyService and optionally provides LoggerService.

💡 Why This Matters

🌍 Real World

Optional providers are useful when you want to add features like logging, caching, or analytics only if they are configured, without forcing all parts of the app to depend on them.

💼 Career

Understanding optional providers helps you write flexible and maintainable NestJS applications that can adapt to different environments and requirements.

1

Create LoggerService class

Create a class called LoggerService with a method log(message: string) that does nothing inside.

NestJS

# Create LoggerService class with log method
# Your code here

Need a hint?

Define a class with a method named log that accepts a message string parameter.

2

Create MyService with optional LoggerService injection

Create a class called MyService with a constructor that optionally injects LoggerService using @Optional() and @Inject() decorators from @nestjs/common. Store it in a private readonly property called logger.

NestJS

import { Optional, Inject } from '@nestjs/common';

export class LoggerService {
  log(message: string) {
    // no operation
  }
}

// Create MyService class with optional LoggerService injection
# Your code here

Need a hint?

Use @Optional() and @Inject(LoggerService) before the constructor parameter logger.

3

Add doWork method using optional logger

In the MyService class, add a method called doWork() that calls this.logger.log('Work done') only if this.logger exists.

NestJS

import { Optional, Inject } from '@nestjs/common';

export class LoggerService {
  log(message: string) {
    // no operation
  }
}

export class MyService {
  constructor(@Optional() @Inject(LoggerService) private readonly logger?: LoggerService) {}

  // Add doWork method that uses logger if available
  # Your code here
}

Need a hint?

Check if this.logger is truthy before calling log.

4

Create module with optional LoggerService provider

Create a NestJS module class called AppModule that provides MyService. Also add LoggerService to the providers array but comment it out to simulate optional provision.

NestJS

import { Module } from '@nestjs/common';

import { Optional, Inject } from '@nestjs/common';

export class LoggerService {
  log(message: string) {
    // no operation
  }
}

export class MyService {
  constructor(@Optional() @Inject(LoggerService) private readonly logger?: LoggerService) {}

  doWork() {
    if (this.logger) {
      this.logger.log('Work done');
    }
  }
}

// Create AppModule with MyService and optional LoggerService provider
# Your code here

Need a hint?

Use @Module decorator with a providers array including MyService and a commented out LoggerService.