That sounds to me like you'll end up getting little benefit from ECS then. Let's say the JavaScript program is iterating over a hundred thousand instances of some Foo class which happens to have 6 properties. You'd ideally want good use of cache, but if your object vector that has all of the Foo objects also happens to have all sorts of instances of other types that have the same field count mixed in there, then you're going to spend a lot of time skipping over those unrelated objects and refreshing the cache.

I know that ECS is treated as a silver bullet by a lot of people, but my experience is that it really only works well when the data you're working with is statically typed so that you can actually partition into arrays where each array does represent a single meaningful type.

That is definitely possible: If it turns out to not make sense then I can at least always go back go the current system where all properties of an object as stored in a single array.

It's not as ECS'ssy as one would hope but it's at least proven technology :D