Expand operator behavior

[kwonoj]

Took example snippet from RxJS 4,

var source = Rx.Observable.return(42)
    .expand(function (x) { return Rx.Observable.return(42 + x); })
    .take(5);

var subscription = source.subscribe(console.log, console.log, ...);

RxJS4

Next: 42
Next: 84
Next: 126
Next: 168
Next: 210
Completed

RxJS Next

RangeError: Maximum call stack size exceeded
    at ExpandSubscriber._next 

[benlesh]

This seems to be more related to take, as when I remove take it works fine.

cc/ @trxcllnt

[kwonoj]

Took a look this bit more, this is behavior of expand. Internally it recursively calling _next() to expand, having 2 problems

• for infinite expand with subscription, recursive call causes stack overflow
• in case of scalar value result via projection at

 if (result._isScalar) {
          this._next(result.value);
 }

it infinitely continues while there isn't terminating condition.

[benlesh]

for infinite expand with subscription, recursive call causes stack overflow

expand is apparently missing the scheduler argument. We should still default to "no scheduler", which means immediate execution (and potential stack overflow).

[kwonoj]

expand is apparently missing the scheduler argument

: compare to RxJS4, yes it does. I'll try changes for this.

(and potential stack overflow)

: I think this is similar case to https://github.com/ReactiveX/RxJS/issues/651, eventually might need some mechanism to avoid stack overflows.

[benlesh]

eventually might need some mechanism to avoid stack overflows.

That mechanism is allowing users to provide a scheduler.

The problem is that any mechanism (scheduling) to avoid stack overflows also: 1. is less performant, and 2. makes debugging with call stacks gnarly. ... those are two of our top goals.

[kwonoj]

I'm closing this issue since scheduler parameter is being tracked in #841 .