Tips for infuriating TSC with Lookup categories

I'm looking for the Typescript compiler (TSC) to throw errors when I make mistakes in signatures.

export class EventEmitter<EventTypes extends { [key: string]: any }> {
    subscribe<Event extends keyof EventTypes>(type: keyof EventTypes, fn: <T extends EventTypes[Event]>(value: T) => any) {

    }
}

export class Data {
    name: string;
}

export class Experiment extends EventEmitter<{ "next": Data, "end": Data[] }> {

}

new Experiment().subscribe("end", (v: Date) => {});

My understanding is that tsc should flag the error in (v: Date) since Date is not of type Data[]. How can this be achieved?

Answer №1

I was making things more complicated than necessary. The solution is actually quite simple.

export class EventEmitter<Events extends { [key: string]: any }> {
    subscribe<EventType extends keyof Events>(type: EventType, callback: (value: Events[EventType]) => any) {

    }
}

Take note of how the type parameter has been updated from keyof EventTypes to EventType. That's the essence of it.

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