map() works for cases where you don't need to compute anything in the callback, you just want to pipeline the elements into another RPC, which is actually a common case with map().
If you want to filter server-side, you could still accomplish it by having the server explicitly expose a method that takes an array as input, and performs the desired filter. The server would have to know in advance exactly what filter predicates are needed.
But in the concrete:
* Looking up some additional data for each array element is a particularly common thing to want to do.
* We can support it nicely without having to create a library of operations baked into the protocol.
I really don't want to extend the protocol with a library of operations that you're allowed to perform. It seems like that library would just keep growing and add a lot of bloat and possibly security concerns.
(But note that apps can actually do so themselves. See: https://news.ycombinator.com/item?id=45339577 )
For any other function accepting a callback, the function on the server will receive an RPC stub, which, when called, makes an RPC back to the caller, calling the original version of the function.
This is usually what you want, and the semantics are entirely normal.
But for .map(), this would defeat the purpose, as it'd require an additional network round-trip to call the callback.