[nevon]

It's not a matter of being smart enough. Since JavaScript is interpreted, the optimization happens at runtime. If the code is executed once and the number of items in the array is small, then it will take more time for the compiler to optimize the code than to naively execute it. Most code falls into this category.

As for whether or not it's possible at all to combine a map and filter into a single loop I guess depends on whether the first operation can have side effects that affect the second operation or the collection that is being iterated over. I don't know the answer, but I would be surprised if there wasn't some hard to detect corner case that prohibits this kind of optimization.