There was even a firm proposal for user-defined value types in JS, but asm.js (and now wasm) took all the air out of the room - though I'm not sure it would have ever gotten implemented in all the browsers anyway, since only perf-sensitive developers care about it.

EDIT: I noticed it's actually mentioned in a footnote in the article - the Typed Objects API was really nice, I had a chance to use it for a prototype.