JS arrays don't store their elements directly, they store references to their elements. (Unless the elements are primitives and the engine is using NaN-boxing o

JS arrays don't store their elements directly, they store references to their elements. (Unless the elements are primitives and the engine is using NaN-boxing or pointer tagging to store those inline, or something like that, which doesn't apply to ECS components generally.) Depending on how the GC works, the elements themselves might still wind up next to each other in memory some of the time. But that is definitely not a pattern that you would expect to hold in general- components will be added and removed over the course of the game, with lots of other stuff happening in between. In C++, the layout of the array elements is actually part of the language semantics. In JS, the language semantics don't even have a way to talk about that layout, and engines in practice don't use the layout you want.