WebSocketSubjectConfig's resultSelector is excepting an not necessary generic type T

[fangpenlin]

RxJS version:

5.4.2

Code to reproduce:

Say if you want to make WebSocket subject emits raw MessageEvent, and you write

import * as Rx from 'rxjs/Rx'

Rx.Observable.webSocket<MessageEvent>({
  url: 'ws://localhost:8080',
  resultSelector: (e: MessageEvent) => e
})

Expected behavior:

It should compiles

Actual behavior:

[ts]
Argument of type '{ url: string; resultSelector: (e: MessageEvent) => MessageEvent; }' is not assignable to parameter of type 'string | WebSocketSubjectConfig'.
  Type '{ url: string; resultSelector: (e: MessageEvent) => MessageEvent; }' is not assignable to type 'WebSocketSubjectConfig'.
    Types of property 'resultSelector' are incompatible.
      Type '(e: MessageEvent) => MessageEvent' is not assignable to type '<T>(e: MessageEvent) => T'.
        Type 'MessageEvent' is not assignable to type 'T'.
(property) resultSelector: (e: MessageEvent) => MessageEvent

Additional information:

I just upgraded to typescript 2.4.1, maybe it has something to do with this change

I can work around the issue either by returning any type from the resultSelector function, or make it a generic function like this

import * as Rx from 'rxjs/Rx'

function resultSelector<T> (e: MessageEvent): T {
  return e as any as T
}

Rx.Observable.webSocket<MessageEvent>({
  url: 'ws://localhost:8080',
  resultSelector
})

But returning any is losing the type check, as we know exactly what type to be expected as it's given as an Rx.Observable.webSocket type argument. And for the second working around, which is even more awkward .

So, maybe a more ideal solution is to make WebSocketSubjectConfig also a generic type, and use the T as the return value from resultSelector?

[kwonoj]

Yes, it seems legit and this seems oversight when we didn't have strict generic checks.

[fangpenlin]

@kwonoj created a bugfix at #2755

[jrmcdona]

I am still facing issues with this.

Here is the code for the WebSocketSubjectConfig.

   this.wsSubjectConfig = {
      url: url,
      closeObserver: {
        next: (e: CloseEvent) => {
          this.socket = null;
          this.connectionObserver.next(false);
        }
      },
      openObserver: {
        next: (e: Event) => {
          this.connectionObserver.next(true);
        }
      },
      resultSelector: (e: MessageEvent) => {
        return this.serialization.handle(e);
      }
    };

here is the error:

src/core/events/rxWebSocket.subject.ts(37,5): error TS2322: Type '{ url: string; closeObserver: { next: (e: CloseEvent) => void; }; openObserver: { next: (e: Event...' is not assignable to type 'WebSocketSubjectConfig'.
  Types of property 'resultSelector' are incompatible.
    Type '(e: MessageEvent) => DataEvent' is not assignable to type '<T>(e: MessageEvent) => T'.
      Type 'DataEvent' is not assignable to type 'T'.

Notice anything I should be doing different?

[benlesh]

Closing this, as resultSelector has been deprecated for some time.

[mfit]

@benlesh but how can the OP's use case be fulfilled (not apply JSON.parseon the incoming messages) ?

Only now I figured out that there are serializer / deserializer arguments in the config of the subject ( https://rxjs.dev/api/webSocket/WebSocketSubjectConfig)