Angular Standalone - How to Inject Service into Service

Question

I'm a longtime Angular dev now trying to learn the standalone version. How do I inject a service into another service without a providers array in the NgModule. Is my only choice using providedIn in the Injectable declaration?

Answer 1

If your service is not providedIn: root, then you must be adding the service to the providers array of some standalone component (in-order to use it). You need to add the second service to the same array, so that the service has its own instance of the second service.

import { Component, inject } from '@angular/core';
import { bootstrapApplication } from '@angular/platform-browser';
import { FirstService } from './app/first.service';
import { SecondService } from './app/second.service';

@Component({
  selector: 'app-root',
  standalone: true,
  providers: [FirstService, SecondService],
  template: `
    <h1>Hello from {{ name }}!</h1>
    <a target="_blank" href="https://angular.dev/overview">
      Learn more about Angular
    </a>
  `,
})
export class App {
  name = 'Angular';
  firstService = inject(FirstService);
}

bootstrapApplication(App);

---

Above method is the best approach, but you have other options also.

Inherit the second service.

First Service:

import { Injectable } from '@angular/core';
import { SecondService } from './second.service';

@Injectable()
export class FirstService extends SecondService {
  constructor(protected http: HttpClient) {
    this.test(http);
  }
}

Second Service:

import { Injectable } from '@angular/core';

@Injectable()
export class SecondService {
  constructor(protected http: HttpClient) {}

  test() {
    console.log('from second service');
  }
}

Initialize the second service as a traditional class

You will have to access the second service as a class and only through the first service, it is not visible in angular DI, and you will have to provider the dependencies to the service (E.g.: HttpClient)

import { Injectable } from '@angular/core';
import { SecondService } from './second.service';

@Injectable()
export class FirstService {
  secondService!: SecondService;
  constructor(protected http: HttpClient) {
    this.secondService = new SecondService(http); // <- create a new instance
    // this exists only inside first service.
  }
}