At the same time, all this copying leads to an immense amount of garbage, which can really slow apps down with GC pauses. I really wished JavaScript had support

At the same time, all this copying leads to an immense amount of garbage, which can really slow apps down with GC pauses. I really wished JavaScript had support for true immutable structures (a la Immutable.js) since these things do add up. In my side project, which is a high performance web app, I was able to get an extra ~20fps by virtually removing all garbage created each frame. And there's a lot of ways to accidentally create garbage. Prime example is the Iterator protocol, which creates a new object with two keys for every step of the iteration. Changing one for loop from for...of back to old-style made GC pauses happen about half as much. But you can't iterate over a Map or Set without the Iterator protocol, so now all my data structures are hand-built, or simply Arrays. I would like to see new language features be designed with a GC cost model that isn't "GC is free!" But I doubt that JavaScript is designed for me and my sensibilities....