Abstract Constructor on Abstract Class in TypeScript

Question

How can I create an abstract constructor for an abstract class?

For example, if I have this abstract class:

export abstract class Foo<T> extends Bar<T> {
  constructor(someParam: any) {
    super();
    this.someObj = someParam;
  }
}

Then it is consumed by this class:

export class FooImpl extends Foo<SomeType> {
}

If I don't override the constructor then the default parameterless constructor will be used on the FooImpl class which will lead to an improper state within the class.

How can I define an abstract constructor or force consumers of the abstract class to override the constructor when Foo<T> is consumed?

EDIT

The exact code I have is below and would appear that this is somehow related to vscode

export abstract class FireStoreDataSource<T> extends DataSource<T> {
  constructor(store: () => Observable<T[]>) {
    super();
  }
}

Then I am extending this class as follows

export class AssessmentCenterDataSource extends FireStoreDataSource<AssessmentCenter> {
}

Then creating an instance in an angular component ngOnInit

ngOnInit() {
  this.dataSource = new AssessmentCenterDataSource(() => this.service.getData());
}

Here in vscode I am getting the compiler error [ts] Expected 0 arguments, but got 1., however when I run ng build it does build correctly

If I do not pass anything to the constructor then the compiler error goes away and I get an error when I run ng build which is error TS2554: Expected 1 arguments, but got 0.

So it looks like this might be an issue with vscode or one of the addons I have installed rather than typescript

I am using vscode insiders 1.26.0 and typescript insiders 3.0.1 along with Angular 6

Answer 1

Actually in Typescript if you don't override the constructor, the constructor is inherited form the base class.

So in your case this would be invalid:

new FooImpl(); // error 

You would be forced to pass the parameters required by the base class:

new FooImpl("") //ok

There is no way to force the derived type to override the constructor, but since the base constructor is inherited it seems to be good enough.

Playground link

Answer 2

Creating a new instance of FooImpl generates an error (when passing no args to the constructor, because the constructor is inherited) thereby forcing the usage of the constructor with args:

const f = new FooImpl();
Expected 1 arguments, but got 0.