[morganherlocker]

> (There are ECS frameworks in JavaScript, which gives you absolutely no control over memory layout and thus completely defeats one of the primary purposes of the pattern.)

While JS does not provide great support for bit packing complex structs, typed arrays give you quite a bit of control over memory layout for simple numeric types, which is what you usually want for optimal data-oriented code anyway. This is a common technique used in fast JS libs for data visualization, ie:

https://github.com/mourner/flatbush

There are also basic operators required for bitarrays, which are useful for ECS and memory-efficient code generally.