generics - TypeScript: self-referencing return type for static methods in inheriting classes

Question

With Polymorphic this in TypeScript 1.7, as I discovered here, we can define a method in a class with a return type of this, and automatically, any classes that extend that class and inherit the methods, will have their return types set to their respective this type. Like so:

class Model {
  save():this {    // return type: Model
    // save the current instance and return it
  }
}

class SomeModel extends Model {
  // inherits the save() method - return type: SomeModel
}

However, what I'm after is to have an inherited static method with a return type referencing the class itself. It's best described in code:

class Model {
  static getAll():Model[] {
    // return all recorded instances of Model as an array
  }

  save():this {
    // save the current instance and return it
  }
}

class SomeModel extends Model {
  // inherits the save() method - return type: SomeModel
  // also inherits getAll() - return type: Model (how can we make that SomeModel?)
}

Perhaps I'll have to think of a different way to implement this, since Polymorphic this in TypeScript 1.7 does not support static methods by design.

EDIT: I guess we'll see how this Github issue wraps up: https://github.com/Microsoft/TypeScript/issues/5863

Answer 1

This is doable in TypeScript 2.0+. By using an inline { new(): T } type to capture this, you'll get what you wanted:

type Constructor<T> = { new (): T }

class BaseModel {
  static getAll<T>(this: Constructor<T>): T[] {
    return [] // dummy impl
  }
  
  /**
   * Example of static method with an argument:
   */
  static getById<T>(this: Constructor<T>, id: number): T | undefined {
    return // dummy impl
  }

  save(): this {
    return this // dummy impl
  }
}

class SubModel extends BaseModel {}

const sub = new SubModel()
const savedSub: SubModel = sub.save()

// Behold: SubModel.getAll() returns SubModels, not BaseModel
const savedSubs: SubModel[] = SubModel.getAll()

Note that getAll still expects no arguments with this typing.

For more information, see https://www.typescriptlang.org/docs/handbook/2/generics.html#using-class-types-in-generics and https://stackoverflow.com/a/45262288/1268016