This is definitely the more meaningful/influential performance benefit of ECS in game development, I believe. JavaScript will not allow for that as you point ou

This is definitely the more meaningful/influential performance benefit of ECS in game development, I believe. JavaScript will not allow for that as you point out. Perhaps a sufficiently crazy JIT might claw some of those benefits back, though? Not sure. But: the lesser but still impactful performance benefit of ECS is the usage of Struct-of-Array vectors for data storage. JavaScript can still ruin that benefit by always accessing all parts and features of an Object every time it touches one, but it is a less likely thing to happen. So, there is a benefit that JavaScript code itself can enjoy. Finally, there is one single "true System" in a JavaScript engine's ECS: the garbage collector. The GC will run through a good part of the engine heap, and you can fairly easily write it to be a batched operation where eg. "all newly found ordinary Objects" are iterated through in memory access order, have their mark checked, and then gather up their referents if they were unmarked. Rinse and repeat to find all live/reachable objects by constantly iterating mostly sequential memory in batches. This can also be parallelised, though then the batch queue needs to become shareable across threads. The sweep of the heap after this is then a True-True System where all items are iterated in order, unmarked ones are ignored, marked ones are copied to their post-compaction location, and any references they hold are shifted down to account for the locations of items changing post-compaction.