Dependency Injection

Inject via constructor

NestJS automatically instantiates and injects services via the constructor. No manual new needed.

@Injectable()
export class UsersService {
  // service logic
}

@Controller('users')
export class UsersController {
  constructor(private usersService: UsersService) {}

  @Get()
  findAll() {
    return this.usersService.findAll();
  }
}

Lazy loading & scopes

By default, services are singletons (one instance for the app). You can also use REQUEST or TRANSIENT scopes for per-request or per-use instances.

Constructor injection in action

Nest reads the constructor parameter types and passes the matching provider.

• @Injectable() marks the class as a provider.
• private readonly users: UsersService both declares and stores the dependency.
• You never write new UsersService() yourself.

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

@Injectable()
export class UsersService {
  findAll() {
    return ['asha', 'ravi'];
  }
}

@Controller('users')
export class UsersController {
  constructor(private readonly users: UsersService) {}

  @Get()
  list() {
    return this.users.findAll();
  }
}

Why DI makes tests easy

Because the controller receives its service instead of creating it, a test can hand it a fake service. No database is needed to test the controller.

Quick check

Quick check: A provider is listed in `providers` but you inject it in another module and get an error. What is missing?

• It must be exported by its module and that module imported
• The class needs `@Controller()`
• Nest only injects providers named `Service`