lambda-jam is a sweet.js macro, λ, which is used to create shorthand, anonymous functions with special argument identifiers. Its functionality is inspired by clojure's shorthand function macro, #(). lambda-jam's real power is derived from its special argument identifiers and the fact that it returns the last expression for you. _1 references the first argument, _2 references the second argument and so on up until _9. Though, the hope is that you're not passing 9 arguments to a function.