Generics: Create instance from abstract class

Question

I am trying to create an instance of a class, but the compile-time error is saying:

Cannot create an instance of the abstract class 'QueueProcess'.

However, I am not creating an instance of it, I am creating an an instance of a class that extends QueueProcess. So, why am I getting this error?

export class Queue<T extends QueueProcess> {

    private _queue: T[] = []

    private async runFirstProcess() {
      let process = new this._queue[0]
    }

}

export abstract class QueueProcess {

}

The code once compiled works fine, it is just throwing that compile-time error.

Answer 1

So, first of all, the line T extends QueueProcess means that T is an instance of QueueProcess, which won't be newable. To access the constructor type, you need T extends typeof QueueProcess.

But that won't work anyway, since QueueProcess itself extends QueueProcess, and since it's abstract, typescript will complain about that. So instead, make T extend a newable function that returns a QueueProcess, eg:

export class Queue<T extends new () => QueueProcess> {

    private _queue: T[] = []

    private async runFirstProcess() {
        let process = new this._queue[0]()
    }

}

export abstract class QueueProcess {

}