we don't have anything similar to `uderscore.invert` available and it will need more lines of code to implement and it's harder to understand what is going on.. one thing is when an operation like `Array#map` or `Array#filter` reduces the amount of code and let you focus on the problem itself.. but in this case the *functional style* is not making things simpler or faster (the imperative implementation does less work). - I don't see the point of changing the implementation..