TypeScript: Cannot infer event argument types from instance of class which is extended from EventEmitter

[vladkrutenyuk]

Hi, I’m trying to create a generic React hook that listens to an EventEmitter3 instance and updates state based on a selected transformation of the event arguments.

Here’s a simplified example:

function useEventEmitterState<
  TState,
  TEmitter extends EventEmitter,
  TEvent extends ReturnType<TEmitter["eventNames"]>[number]
>(
  emitter: TEmitter,
  event: TEvent,
  select: (...args: any[]) => TState,
  init: (() => TState) | TState
) {
  const [state, setState] = useState<TState>(init);

  useEffect(() => {
    if (!emitter) return;

    const listener = (...args: any[]) => {
      setState(select(...args));
    };

    emitter.on(event, listener);
    return () => emitter.off(event, listener);
  }, [emitter, event]);

  return state;
}

Problem:

TypeScript cannot infer the argument types for select based on the event name TEvent and the emitter instance TEmitter.

Currently I have to use any[] for ...args in select and the internal listener.

I would like a way to type select so that:

const state = useEventEmitterState(emitter, "smthChanged", (arg1, arg2) => { ... }, ...);

TypeScript automatically infers arguments for event name.

Request:

Could the EventEmitter3 typings support inferring event argument types from the generic EventMap of the emitter instance? Something similar to:

interface MyEvents {
  fileSelected: [File];
  otherEvent: [string, number];
}
class Example extends EventEmitter<MyEvents> {
  selectedFile: File | null = null;
}
const emitter = new Example();

function MyComponent() { 
  // select callback should infer args based on "fileSelected"

  const selectedFile = useEventEmitterState<File | null>(emitter, "fileSelected", (file) => file, emitter.selectedFile);
  const someOtherState = useEventEmitterState<string>(emitter, "otherEvent", (arg1,arg2) => `${arg1}${arg2}`, "");

  return <div>hi</div>
}

Right now, using only ReturnType<TEmitter["eventNames"]>[number] is insufficient, because TS loses the argument types.

Thanks!