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

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

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
}

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

Hint

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

Practice

1. What is the main purpose of using @Optional() in a NestJS provider?

easy

A. To make a provider always required and throw error if missing

B. To mark a dependency as optional, so it may be undefined if not provided

C. To automatically create a new instance of the provider

D. To disable dependency injection for that provider

Solution

1. Step 1: Understand the role of @Optional()

   @Optional() tells NestJS that the dependency might not be available and should not cause an error if missing.

2. Step 2: Compare with required providers

   Without @Optional(), NestJS throws an error if the provider is not found. With @Optional(), it injects undefined instead.

3. Final Answer:

   To mark a dependency as optional, so it may be undefined if not provided -> Option B

4. Quick Check:

   @Optional() = marks provider optional [OK]

Hint: Optional means provider can be missing without error [OK]

Common Mistakes:

• Thinking @Optional() makes provider required
• Assuming @Optional() creates new instances
• Believing @Optional() disables injection

2. Which of the following is the correct way to inject an optional provider in a NestJS constructor?

easy

A. constructor(@Optional() private readonly service: MyService) {}

B. constructor(private readonly service: MyService) {}

C. constructor(@InjectOptional() private readonly service: MyService) {}

D. constructor(@Optional private service: MyService) {}

Solution

1. Step 1: Recall the correct decorator syntax

   The correct decorator to mark a provider optional is @Optional() with parentheses.

2. Step 2: Check each option

   constructor(private readonly service: MyService) {} lacks @Optional, so provider is required. constructor(@InjectOptional() private readonly service: MyService) {} uses a non-existent decorator. constructor(@Optional private service: MyService) {} misses parentheses on @Optional.

3. Final Answer:

   constructor(@Optional() private readonly service: MyService) {} -> Option A

4. Quick Check:

   @Optional() decorator syntax requires parentheses [OK]

Hint: Use @Optional() with parentheses before the parameter [OK]

Common Mistakes:

• Forgetting parentheses on @Optional()
• Using wrong decorator names
• Not marking provider optional in constructor

3. Given the following code, what will be the output when MyService is NOT provided in the module?

constructor(@Optional() private readonly myService?: MyService) {}

useService() {
  return this.myService ? 'Service available' : 'Service missing';
}

medium

A. "Service missing"

B. "Service available"

C. Runtime error: Cannot read property of undefined

D. Compilation error due to missing provider

Solution

1. Step 1: Understand @Optional() effect when provider missing

   When MyService is not provided, the injected value is undefined, not an error.

2. Step 2: Analyze the useService method

   The method checks if myService exists. Since it's undefined, it returns "Service missing".

3. Final Answer:

   "Service missing" -> Option A

4. Quick Check:

   Missing optional provider = undefined, so returns "Service missing" [OK]

Hint: Optional provider missing means undefined, check before use [OK]

Common Mistakes:

• Expecting runtime error when provider missing
• Assuming provider is always injected
• Ignoring undefined check before usage

4. Identify the error in this NestJS service constructor using an optional provider:

constructor(@Optional() private readonly service: MyService) {
  this.service.doSomething();
}

medium

A. Missing @Inject decorator for MyService

B. Service should not be private readonly

C. Calling method on possibly undefined service without check

D. Incorrect use of @Optional() decorator without parentheses

Solution

1. Step 1: Check usage of optional provider

   @Optional() means service can be undefined if not provided.

2. Step 2: Analyze method call in constructor

   Calling this.service.doSomething() without checking if service exists can cause runtime error.

3. Final Answer:

   Calling method on possibly undefined service without check -> Option C

4. Quick Check:

   Always check optional provider before use [OK]

Hint: Check if optional provider exists before calling methods [OK]

Common Mistakes:

• Calling methods on undefined optional providers
• Assuming @Optional() means always injected
• Ignoring runtime errors from missing checks

5. You want to inject an optional logging service in your NestJS component. If the logging service is missing, your component should still work without errors. Which approach correctly implements this behavior?

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

  logMessage(msg: string) {
    // What should you do here?
  }
}

hard

A. Call this.logger.log(msg) directly without checks

B. Throw an error if this.logger is undefined

C. Remove @Optional() to ensure logger is always injected

D. Use if (this.logger) this.logger.log(msg); to check before calling

Solution

1. Step 1: Understand optional injection with @Optional()

   The logger may be undefined if not provided, so direct calls risk runtime errors.

2. Step 2: Implement safe usage in method

   Check if this.logger exists before calling log() to avoid errors.

3. Final Answer:

   Use if (this.logger) this.logger.log(msg); to check before calling -> Option D

4. Quick Check:

   Check optional provider exists before use to avoid errors [OK]

Hint: Always check optional provider before method calls [OK]

Common Mistakes:

• Calling methods without existence check
• Throwing errors instead of handling missing provider
• Removing @Optional() and causing injection errors