Angular：使用构造函数注入服务的替代方法

Question

All examples I found, injected a service like this:

export class TestComponent {

  constructor(
    private dataService: DataService,
    private dataService2: DataService2,
    private dataService3: DataService3,
    // ...
    // ...
    // ...
    // ...
    // ...
    // ...
    // ...
  ) {
        // Do something
    }
}

However, this list can get very long and confusing if you inject several services and also use the constructor method to initialize something. So the question is:
Can I write this alternatively?

export class TestComponent {

  private dataService = new DataService();
  private dataService2 = new DataService2();
  private dataService3 = new DataService3();

  constructor() {
        // Do something
  }
}

Answer 1

It depends on what exactly your service need to do. If it just a bunch of usefull methods, you can do it, but in every component where u write somthing like this private dataService = new DataService(); u will get new instance of service.

on the other hand, in my work we often use Services as data storages, to query something from it, with this new keyword u will instantiate it and won't get any data that were passed from another component.

Anyways DI is a great part of Angular ecosystem. So u can declare it into providers on modules (before that separate your components to rely on some module) that are actually using them to lower its quantity into app module. Because if you inject som many stuff into 1 component constructor better to separate some functionality, because it may become really ugly.