返回实例类型<t>或未定义</t>

Question

I'm creating a registry where if I pass in a class to a Map as a key, it will return an instance of that class as a value.

class Car {}
class HondaCar extends Car{}

type Newable = { new (...args: unknown[]): unknown };

export default class CarRegistry {
  private cars = new Map<NewableFunction, Car>();

  register(instance: Car) {
    this.cars.set(instance.constructor, instance);
  }

  get<T extends Newable>(type: T) {
    if (!this.cars.has(type)) return undefined;
    return this.cars.get(type) as InstanceType<T>;
  }
}

The expected functionality is something like:

const registry = new CarRegistry()
registry.register(new HondaCar())
car = registry.cars.get(HondaCar)

The issue, however, is that I want register.cars.get to be able to return undefined . For some reason, no matter what combination of types I try for get , it's not possible in this context to return undefined . Here's the signature TypeScript generates for me:

(method) CarRegistry.get<typeof HondaCar>(type: typeof HondaCar): HondaCar

The types are completely correct, which is great, but I want this function to return HondaCar | undefined HondaCar | undefined . This is necessary because there is a large chance that the car has not yet been registered, and I want the caller to account for that.

Is there any way I can return InstanceType<T> OR undefined ?

Answer 1

This can happen if strictNullChecks are disabled. Enabling "strict": true in my TSConfig resolved my issue.