[Benvie]

You need to do things like not dispatch to valueOf and toString for coercion. You basically just need to get rid of dynamic coercion dispatch, and do what you said (annotate types) and that's all that's really needed. But the ECMAScript runtime is pretty substantial so having a minimized subset of that runtime would make it possible to self-host most of the higher level interpreter in ES6-like code instead of having most of it implemented in ES3 (as it is now). This makes a significant difference in verbosity of the code. As converted to ES6, I estimate the code would be half of what it is now (currently it contains ~23k lines of ES3 code, 7k lines of ES6 code, so 30k total).