its a nice write up, however you still don't necessarily have the composition flow like you do when using ramda. Plus the libraries such as ramda can always dispatch to native methods and provide convenience wrappers to create this compositions. Again you can do everything yourself by using the above methods and groupings but then why re-invent the wheel each time when you already have utility libraries there seems very little win/gain.
I guess you haven't used hapi, express was nice for building simple solution but as this wrapper points out in the readme it's not great for complex situations without a lot more foundational work. When we moved to hapi we were able to make complex e-commerce system without any problems or mucking around so i was simply stating s switch to a more robust framework. Take the advice with a grain of salt :)
its the NOT bitwise operator which simple takes -(n + 1) where n is the number to the right of the operator. then !! is a way of coercing to boolean so essentially the returned value is either true or false depending if letter exists in the array instead of returning the index or -1 if it doesnt exist.