[no-useless-constructor] constructor with super(someArgs) is not useless in Angular

[JounQin]

Repro

{
  "rules": {
    "@typescript-eslint/no-useless-constructor": 2
  }
}

class A {
  protected http: HttpClient
  constructor(protected injector: Injector) {
    this.http = this.injector.get(HttpClient)
  }
}

class B implements A {
  // this should not be considered as useless due to DI
  constructor(injector: Injector) {
    super(injector)
  }
}

Expected Result

No error report

Actual Result

Error

Versions

package	version
@typescript-eslint/eslint-plugin	2.6.0
@typescript-eslint/parser	2.6.0
TypeScript	3.6.4
ESLint	6.6.0
node	13.0.1
npm	6.12.0

[JounQin]

We can use following workaround temporarily:

class B implments A {
  constructor(protected injector: Injector) {
    super(injector);
  }
}

But if A declare it as private, we can only change it to another property name like:

class A {
  private http: HttpClient
  constructor(private injector: Injector) {
    this.http = this.injector.get(HttpClient)
  }
}

class B implements A {
  constructor(private _injector: Injector) {
    super(_injector)
  }
}

[bradzacher]

Why do you keep writing class B implements A?
That's not valid code?

---

Could you please explain why you need the constructor on the child class?

Unless I'm mistaken, this is perfectly valid code, which does exactly the same thing, without the useless constructor:

class A {
  protected http: HttpClient
  constructor(protected injector: Injector) {
    this.http = this.injector.get(HttpClient)
  }
}

class B extends A { }

[JounQin]

Oh sorry, it should be extends, it's a typo.

B's constructor is required in favor of DI system in Angular.

[bradzacher]

Have you got some docs on why the constructor is explicitly needed on the child?
I haven't used angular since like 1.6, so I don't know how it works.

Unless I'm mistaken it shouldn't need it explicitly, as the child should implicitly inherit the parent's constructor.

[JounQin]

@bradzacher Just take https://stackblitz.com/edit/angular-gh3sf4 as an example.

[bradzacher]

Please be careful with your @ tagging. You tagged the wrong account.
Github has a crappy implementation, and doesn't put my name first in the autocomplete list.

---

Looking into this, I have no idea why it's required. I think angular statically analyses the file by itself? I'm not sure, the custom compiler is a black box to me.

We don't want to encode the semantics of angular into typescript.
The fact that the constructor is required by the angular compiler is an implementation detail of the framework.

If you'd like this functionality, I'd suggest getting a rule created in a plugin like eslint-plugin-angular.

[JounQin]

@bradzacher Thx for your patients. I will open a feature request issue at angular-eslint then.