An enormous amount of the shavings and inlining that the Closure compiler is capable of assumes that you write your code according to rules that reduce ambiguity and dynamic features in your code. Variable names get mangled, so things like using string names for properties is out the window, which means invoking methods by string names is out as well. It uses objects for namespaces so things like `Shape.Quad.Square` get renamed to `Shape$Quad$Square` before getting minified to some one-letter variable, so don't use `this` outside of constructors or prototype methods.

There's a larger list of restrictions at https://developers.google.com/closure/compiler/docs/limitati...

If you do write code obeying these rules (check out the closure library), or you use something like ClojureScript that generates straightforward, monomorphic, closure-compiler-compatible js, the payoff in both file size and parse/execution performance is pretty great.