Observable constructor doesn't require any parameter

[martinsik]

RxJS version: 5.5.3

Code to reproduce:

import { Observable } from 'rxjs/Observable';

const o = new Observable();
// const o = Observable.create();

o.subscribe();

Expected behavior:

Throw compilation error or run fine and do nothing.

Actual behavior:

Compiles fine but throws error when run.

Additional information:

I've seen this question on StackOverflow at least three times where people misunderstood the difference between Subject and Observable classes. They created an instance of Observable and then didn't know what to do next.

What's confusing is that you can create an Observable with just new Observable() (or Observable.create()) because the subscribe callback is optional but when they try to subscribe it throws an error:

/Users/.../node_modules/rxjs/Observable.js:165
                throw sink.syncErrorValue;
                ^

TypeError: Cannot read property 'subscribe' of undefined
    at Observable._subscribe (/Users/.../node_modules/rxjs/Observable.js:231:27)
    at Observable._trySubscribe (/Users/.../node_modules/rxjs/Observable.js:172:25)
    at Observable.subscribe (/Users/.../node_modules/rxjs/Observable.js:160:65)
    at Object.<anonymous> (/Users/.../general/subscribe_01.js:6:3)
    at Module._compile (module.js:573:30)
    at Object.Module._extensions..js (module.js:584:10)
    at Module.load (module.js:507:32)
    at tryModuleLoad (module.js:470:12)
    at Function.Module._load (module.js:462:3)
    at Function.Module.runMain (module.js:609:10)

The error is thrown because this line by default checks if this.source exists:

sink.add(this.source ? this._subscribe(sink) : this._trySubscribe(sink));

but then it tries to use this.source anyway:

return this.source.subscribe(subscriber);

So maybe the constructor should require either the subscribe callback or a source to be always set. For example like this:

constructor(subscribeOrSource: (this: Observable<T>, subscriber: Subscriber<T>) => TeardownLogic | Observable<any>) {

[kwonoj]

If I recall correctly, this was due to signature incompatibility with Subejct as same as signature of static create (which accepts general Function only, cannot have strong types). May need to revisit if that's still cases, but since Subject doesn't require param on construct not sure unless we bend over some types.

[kwonoj]

I still have to check once again and see if that's true though.

[martinsik]

So at the end this might be related to this: #2004

[martinsik]

I checked it again today with [email protected] and it doesn't throw any error and does nothing in both cases which is fine I think so I'm going to close this.

https://stackblitz.com/edit/rxjs-7diu1c?file=index.ts