Can't dynamically map type from class private field

Suggestion

Support private fields in mapped types.

🔍 Search Terms

typescript, private field, mapped types

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Include information about private fields types inside class interface.

📃 Motivating Example

TypeScript have dynamic mapped types which is very powerful feature. But currently can't used with private fields. So its hard to migrate to those.

💻 Use Cases

Working example without private field:

interface Runnable {
    (): void;
}

interface Consumer<T> {
    (data: T): void;
}

class EventEmitter<T> {
    emit(data: T): void {
        return void 0;
    }

    subscribe(consumer: Consumer<T>): Runnable {
        const unsubscribe = () => void 0;

        return unsubscribe;
    }
}

class Host {
    private readonly events = {
        init: new EventEmitter<Record<string, number>>(),
        contextUpdate: new EventEmitter<Record<string, string>>(),
        inactivity: new EventEmitter<boolean>(),
    } as const;

    on<T extends HostEventType>(
        type: T,
        consumer: HostEventConsumer<T>,
    ): Runnable {
        return this.events[type].subscribe(consumer);
    }
}

type HostEventType = keyof Host['events'];

type HostEventConsumer<T extends HostEventType> =
    Host['events'][T] extends EventEmitter<infer U> ? Consumer<U> : never;

const host = new Host();

// Here data is mapped to Record<string, number>
host.on('init', (data) => console.log({ data }));

Not working example with private field (expecting to work in future):

interface Runnable {
    (): void;
}

interface Consumer<T> {
    (data: T): void;
}

class EventEmitter<T> {
    emit(data: T): void {
        return void 0;
    }

    subscribe(consumer: Consumer<T>): Runnable {
        const unsubscribe = () => void 0;

        return unsubscribe;
    }
}

class Host {
    readonly #events = {
        init: new EventEmitter<Record<string, number>>(),
        contextUpdate: new EventEmitter<Record<string, string>>(),
        inactivity: new EventEmitter<boolean>(),
    } as const;

    on<T extends HostEventType>(
        type: T,
        consumer: HostEventConsumer<T>,
    ): Runnable {
        return this.#events[type].subscribe(consumer);
    }
}

type HostEventType = keyof Host['#events'];

type HostEventConsumer<T extends HostEventType> =
    Host['#events'][T] extends EventEmitter<infer U> ? Consumer<U> : never;

const host = new Host();

// Here data should be mapped to Record<string, number>
host.on('init', (data) => console.log({ data }));

issue にある動作する TypeScript の例と動作しない TypeScript の例から始め、keyof と indexed access が private #events フィールドをどのように扱うかに焦点を当てます。private フィールドに対する意図された mapped-type の動作を特定し、両方の例に照らして検証します。private フィールドの例で、JavaScript のランタイム動作を変更せずに、示されているとおりにイベント payload の型が推論されれば完了です。