It faithfully models the original code. The idea is that if you want to transform `!0` to `true`, or if you want to obfuscate, etc. you can always plug an additional tool.
The 5% gains over minified code are impressive given that BinJS performs no AST manipulation.
The BinJS prototype will be written in pure javascript I assume to speed its adoption?
For the moment, it's JS + Rust, because Rust is better than JS when you keep refactoring Big Hairy Data Structures. However, once the data structures stabilize we're planning to shift it entirely to JS.