Remove mergeScan or provide better documentation for it

I have a problem with mergeScan, namely, I can’t think of a single situation in which it’d be needed. The categories I see of problems almost needing mergeScan are:

• Internal state is needed, but an inner observable is unnecessary (just use scan). The example in the docs is in this category
• There is some operation (probably async) that requires an inner observable, but no internal state is needed (just use mergeMap). For instance, successive async calls.
• There is an operation that requires both an async inner observable and internal state BUT is vulnerable to race conditions. In this case, mergeScan falls flat on its face, as the accumulator passed in could be stale by the time the inner observable returns. In this case, it’s better to just For example, see this (entirely contrived) example where I sum the weight of the first 10 Pokémon: http://jsbin.com/qosiwenaqi/edit?js,console

In short, I don’t see a use for mergeScan that isn’t better served by another operator and worse, it can cause unintended race conditions.

So, as a “What do we do about this issue”, I’d like either:

• Remove the mergeScan operator from RxJS v5

or

• Clarify intended uses of mergeScan in the docs so silly folks don’t open elaborate issues demanding its removal

(Both @robwormald and @rgbkrk have discussed this with me in various Slacks so I’m cc’ing them here)

[ghetolay]

Just so you know I’m using mergeScan to do progressive image display. I download and decode the image by chunk so I need the last chunk to compute the next and all this is done async using web worker.

But in fact I’m using concatScan because I set concurrent parameter to 1 that’s why I’m not facing the race condition problem enounced.

[gilesbradshaw]

Please don’t get rid of it very useful as a concatScan with concurrency = 1