The observer return value effects how an Observable emits:
If an observer returns undefined the Observable invokes the next observer immediately.
If an observer returns a Promise, the Observable awaits the return value then continues as
described here. The emitter must then await the Promise returned by Observable.emit.
Any other return value is returned by Observable.emit and subsequent observers do not see emission.
A callback function for observables.
The observer return value effects how an Observable emits:
If an observer returns undefined the Observable invokes the next observer immediately.
If an observer returns a Promise, the Observable awaits the return value then continues as described here. The emitter must then await the Promise returned by Observable.emit.
Any other return value is returned by Observable.emit and subsequent observers do not see emission.