The innermost rendering loop in the JS seems to create and destruct an array instance for every single pixel iteration (see https://github.com/dmaynard/chaos-screen-saver/blob/master/s... ). I guess that this could be potentially optimized away by the JIT, but it will make things slower or at least less predictable.
That's kind of an underrated aspect of these comparisons - while you absolutely can work around Javascript's weird performance cliffs and avoid putting pressure on the garbage collector, you have to fight the language at every turn because so many idiomatic JS patterns are inherently slow or flood the GC. You may find idiomatic JS easier to work with than something like Rust, but Rust is much easier to work with than the narrow and loosely defined subset of JS that you have to stick to for optimal performance. Taken to its limit you end up more or less writing asmjs by hand.