How to Use @Injectable Decorator in NestJS: Simple Guide

In NestJS, use the @Injectable decorator to mark a class as a provider that can be injected into other classes via dependency injection. This decorator tells NestJS to manage the class lifecycle and make it available for injection in constructors.

Syntax

The @Injectable decorator is placed above a class declaration to mark it as a provider. This allows NestJS to create and inject instances of this class where needed.

• @Injectable(): Marks the class as injectable.
• Class declaration: The service or provider class you want to inject.

typescript

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

@Injectable()
export class MyService {
  // service methods and properties
}

Example

This example shows a simple service class marked with @Injectable and how it is injected into a controller using constructor injection.

typescript

import { Injectable, Controller, Get } from '@nestjs/common';

@Injectable()
export class GreetingService {
  getGreeting(): string {
    return 'Hello from GreetingService!';
  }
}

@Controller()
export class AppController {
  constructor(private readonly greetingService: GreetingService) {}

  @Get()
  getHello(): string {
    return this.greetingService.getGreeting();
  }
}

Output

When you visit the root URL, the response will be: "Hello from GreetingService!"

Common Pitfalls

Common mistakes when using @Injectable include:

• Forgetting to add @Injectable() on the service class, which causes NestJS to throw an error when injecting.
• Not registering the provider in a module's providers array, so NestJS doesn't know about it.
• Trying to inject a class that is not decorated with @Injectable or not provided in any module.

typescript

/* Wrong: Missing @Injectable decorator */
export class WrongService {}

/* Right: Add @Injectable decorator */
import { Injectable } from '@nestjs/common';

@Injectable()
export class RightService {}

Quick Reference

Remember these tips when using @Injectable in NestJS:

• Always decorate service classes with @Injectable().
• Register providers in the module's providers array.
• Use constructor injection to access services.
• @Injectable enables NestJS to manage the class lifecycle and dependencies.

Key Takeaways

Use @Injectable() to mark classes as providers for dependency injection in NestJS.

Always register injectable classes in a module's providers array.

Inject services via constructor parameters to use them in other classes.

Forgetting @Injectable or module registration causes injection errors.

The decorator enables NestJS to manage service lifecycles automatically.