It's actually not exactly 50K, and likely varies across Chrome/V8 versions, operating system and/or machine architecture. On my browser the threshold is around 22500 elements. For a quick test, see [1].

The difference in V8 appears to be based on whether or not the array is stored in "large object space" [2], which seems to be used when the object is somewhere between 64kb and 128kb [3] (once again can depend on lots of factors). I wonder how many other optimizations depend on that?

[1]: http://www.lonniebest.com/BadShiftPerformance/ [2]: https://issues.chromium.org/issues/42202676 [3]: https://github.com/danbev/learning-v8/blob/master/notes/heap...