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There's no need to have a restricted Javascript. All you have to do is to write code in ES6 with the type annotations and the entire program will be fully type-inferable (tho the inferring part may take a while because you probably need to do something like a Cartesian product for duck typing). Once you do that, you can do a lot of optimizations.

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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).

Actually, you need to not use a few more features too, like not dynamically attaching properties to objects at run time, and dynamically creating new clasess... but the point is, you don't need to define a new language when all you have to do is to use only a subset.

Well RPython is just that: a restricted subset of Python (hence the name). Similarly, asm.js is a (small) subset of ECMAScript targeted at compile-to-js code generators. It's just about specifying what the subset contains, not inventing a new language.

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