Generic should be output class in typescript in factory method

Question

I would like to pass a generic so i can pre-type my returned class.

public createVehicle<O extends AbstractVehicle>(
  type: string
): O {
  switch (type) {
    case "Car":
      return new Car(param1, param2);
    case OfferType.Sale:
      return new Bike(param3, param4)    
    default:
      throw new Error("Vehicle type not supported");
  }
}

however this gives me:

Type 'Car' is not assignable to type 'O'.
  'Car' is assignable to the constraint of type 'O', but 'O' could be instantiated with a different subtype of constraint 'AbstractVehicle'

which i kind of understand as it can be something like AbstractVehicle & {} but shouldn't this just be an easy thing to do?

Tried for hours without solution, it keeps throwing that error. any suggestions to keep this simple and readable?

Answer 1

Your function is not generic over its return type. You should just write

createVehicle(type: string): Car | Bike

or

createVehicle(type: string): AbstractVehicle

and call it as createVehicle("Car") as Car not createVehicle<Car>("Car").

If anything, your function is generic over its argument type, and you can infer the return type from that:

public createVehicle<T extends "Car" | "Bike">(type: T): T extends "Car" ? Car : T extends "Bike" ? Bike : never

This will infer the correct result type for the call createVehicle("Car") (i.e. typeof createVehicle<"Car">("Car") = Car), but for createVehicle(unknownType) it will still infer the union.