Importing a Module vs. Providing a Service in NestJS

NestJS is a powerful framework for building scalable Node.js applications. One of the key concepts in NestJS is dependency injection, which allows you to manage and share instances of services across different parts of your application efficiently. However, understanding when to import a module versus when to provide a service directly in a module's providers array is crucial to avoiding unexpected behaviors such as duplicate instances, memory leaks, and inconsistencies.

The Key Difference

Importing a Module

When you import a module in NestJS, you get the same instance of any provider that is exported by that module. This means that if a provider is defined in a module and exported, any other module that imports this module will reuse the same instance of the provider.

Providing a Service Directly

If you instead provide a service directly in a module's providers array without importing the module that originally defined it, NestJS will create a new instance of that service. This can lead to unnecessary memory usage and unexpected behavior because multiple instances of what should be a singleton service may be created.

Example: Importing vs. Providing a Service

Let's look at an example to illustrate the difference between these two approaches.

Traditional JavaScript Approach (Without Dependency Injection)

Before diving into NestJS-specific behavior, let's consider how this works in plain JavaScript:

// logger.js
class Logger {
  constructor() {
    if (!Logger.instance) {
      this.logs = [];
      Logger.instance = this;
    }
    return Logger.instance;
  }

  log(message) {
    this.logs.push(message);
    console.log("LOG:", message);
  }
}

module.exports = new Logger();

// app.js
const logger = require("./logger");

function main() {
  logger.log("Application started");
  anotherFunction();
}

function anotherFunction() {
  const anotherLogger = require("./logger");
  anotherLogger.log("Another function called");
  console.log("Same instance:", logger === anotherLogger); // true
}

main();

Since logger.js exports a single instance, every file that requires it gets the same instance, ensuring consistent state. This is similar to how importing a module in NestJS works.

Correct Approach: Importing a Module in NestJS

// logger.module.ts
import { Module } from '@nestjs/common';
import { LoggerService } from './logger.service';

@Module({
  providers: [LoggerService],
  exports: [LoggerService],
})
export class LoggerModule {}

// app.module.ts
import { Module } from '@nestjs/common';
import { LoggerModule } from './logger.module';
import { AppService } from './app.service';

@Module({
  imports: [LoggerModule],
  providers: [AppService],
})
export class AppModule {}

// app.service.ts
import { Injectable } from '@nestjs/common';
import { LoggerService } from './logger.service';

@Injectable()
export class AppService {
  constructor(private loggerService: LoggerService) {}
}

Here, AppService receives the same instance of LoggerService that was created in LoggerModule. This is the recommended way to share a service across modules.

Incorrect Approach: Providing the Service Again

If instead of importing LoggerModule, we declare LoggerService in AppModule's providers array, we get a new instance of LoggerService, which is usually undesirable:

@Module({
  providers: [LoggerService, AppService], // Creates a new instance of LoggerService!
})
export class AppModule {}

Now, AppService gets a different instance of LoggerService, separate from any other modules that might also use LoggerService. This can lead to inconsistencies, especially if LoggerService maintains any internal state.

Why Importing a Module is Preferred

• Instance Reuse: Importing the module ensures all parts of the application use the same instance of a service.
• Memory Efficiency: Creating multiple instances unnecessarily increases memory usage.
• Avoids Hard-to-Debug Issues: When using multiple instances, class properties may not behave as expected because they belong to different objects.

Conclusion

When working with NestJS, always prefer importing a module instead of providing a service in a providers array unless you explicitly need multiple instances. By importing the module, you ensure that all parts of the application reuse the same instance of a provider, leading to better performance, less memory usage, and fewer bugs. NestJS’s module system is designed to encourage modularity while efficiently managing dependencies, and understanding this distinction is key to writing maintainable and scalable applications.