Yeah--JS seems to allow enough weird stuff with arrays that fast paths for when things aren't weird seem inevitable. I admit when I clicked I still found V8 had more implementations than I expected, haha.

What they do with strings is kind of neat, because if you do a lot of concats in a row (potentially an O(n^2) series of operations done the naïve way), they defer actually concatenating all the data until you try to get a character out or such (the string is just a "ConsString" pointing to the source strings): https://gist.github.com/mraleph/3397008

It's also impressive how much dynamic instrumentation they can do. Beyond finding hot code and guessing types, they're even e.g. using it to guess which garbage will be long-lived (for "pretenuring" to avoid needing to copy it).

There's a limit to how much clever stuff you can do implicitly since you don't want to impose big costs for code that's doing everything right; something like QList isn't that adventurous but it's also clearly wrong when you want to compactly store a bunch of small structs in memory. And like the linked post on V8's thing notes, when you patch over a problem in one specific case the 'fix' can be fragile. Still, interesting area.