My only complaint is it is written in Haxe instead of ES6. It would make sense to code it in Haxe if it were to be ported to PHP, C++ and the likes but it appears to be strictly for web audio
It's not like haxe is some kind of crazy offshoot language, it looks pretty damn close to typescript and flow (But to me this looks closer to "regular javascript" than most typescript and flow does).
With the exception of the `@:expose` and `@:keep` stuff, there isn't anything i've seen yet just browsing through the source that I don't understand (as a javascript developer), and i've never seen the language before in my life.
It's not as cool (and backed by big Californian co) as JS/TS, but Haxe really grows on you when you start mastering it.
In many cases, compiler metadata is specific to a given target. E.g., they can be quite common on extern definitions. This allows the Haxe language to stay small, while providing flexibility for better target interoperability.
In this case, @:expose will expose certain methods to the global namespace, and @:keep will prevent the compiler from removing code with its dead code elimination functionality. Both of these are very useful for creating and maintaining js libraries, regardless of whether or not the library is also cross-platform.