A Guide on Dependency Injection in NestJS

Dependency Injection (DI) is a design pattern that allows classes to define their dependencies without creating them. It's a fundamental concept in building scalable and maintainable applications. NestJS, being a TypeScript framework for building efficient and scalable server-side applications, heavily utilizes dependency injection. Here's a guide on how to implement Dependency Injection in NestJS:

1. Understanding Providers:

   In NestJS, services, repositories, and other classes that are managed by the NestJS IoC (Inversion of Control) container are called providers. Providers are classes decorated with @Injectable() decorator.

                   
                       import { Injectable } from '@nestjs/common';
   
                       @Injectable()
                       export class ExampleService {
                         // Service logic here
                       }                    
                   
               

2. Registering Providers:

   To make NestJS aware of providers, they need to be registered in modules. Modules encapsulate related functionality and serve as a way to organize code in NestJS applications.

                   
                       import { Module } from '@nestjs/common';
                       import { ExampleService } from './example.service';
                       
                       @Module({
                         providers: [ExampleService],
                         exports: [ExampleService], // If this service needs to be accessible in other modules
                       })
                       export class ExampleModule {}                    
                   
               

3. Constructor Injection:

   Dependency injection is primarily achieved through constructor injection in NestJS. When a class requires an instance of another class, you simply declare it as a parameter in the constructor.

                   
                       import { Injectable } from '@nestjs/common';
                       import { ExampleService } from './example.service';
                       
                       @Injectable()
                       export class AnotherService {
                         constructor(private readonly exampleService: ExampleService) {}
                       
                         // Use exampleService methods here
                       }                    
                   
               

4. Using Dependencies:

   Once a dependency is injected, you can use it within the class just like any other property or method.

                   
                       import { Injectable } from '@nestjs/common';
                       import { ExampleService } from './example.service';
                       
                       @Injectable()
                       export class AnotherService {
                         constructor(private readonly exampleService: ExampleService) {}
                       
                         someMethod() {
                           return this.exampleService.someFunction();
                         }
                       }                    
                   
               

5. Custom Providers:

   You can define custom providers by implementing interfaces or extending classes. This is particularly useful for integrating third-party libraries or abstracting functionality.

                   
                       import { Injectable } from '@nestjs/common';
                       import { CustomLibrary } from 'custom-library';
                       
                       @Injectable()
                       export class CustomProvider {
                         constructor(private readonly customLibrary: CustomLibrary) {}
                       
                         someMethod() {
                           return this.customLibrary.someFunction();
                         }
                       }                    
                   
               

6. Scope of Providers:

   Providers in NestJS can be either SINGLETON or TRANSIENT. By default, they are SINGLETON, meaning the same instance is shared across the entire application. You can make a provider TRANSIENT, which creates a new instance each time it's requested.

                   
                       import { Injectable, Scope } from '@nestjs/common';
   
                       @Injectable({ scope: Scope.TRANSIENT })
                       export class TransientService {
                         // Transient service logic
                       }                    
                   
               

7. Circular Dependencies:

   Avoid circular dependencies as much as possible, as they can lead to runtime errors. If you encounter a circular dependency, consider refactoring your code to remove it.

Dependency Injection is a powerful technique for managing dependencies in NestJS applications. By following the principles outlined in this guide, you can build scalable, maintainable, and testable applications with ease.

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