Depends where. I.e. I spent 3 years on nodejs project, and first require was either lodash or underscore, bringing in the usual suspects of map, reduce, filter

Depends where. I.e. I spent 3 years on nodejs project, and first require was either lodash or underscore, bringing in the usual suspects of map, reduce, filter along with some sort of pipelining operator. I spent 6 months on C# project using RX, and the code was quite functional, with anonymous functions and closures everywhere. Of course, once you move away from mutable data-structures and variables as default (i.e. clojure, erlang), or even to limitting side effects with type system (haskell), you are entering a niche.