transducers.js

map = function(f) {
  return function(rf) {
    return function(acc, elem) {
      return rf(acc, f(elem));
    };
  };
};

square = function(n) {return n*n;};
add = function(a,b) {return a+b;};

[1,2,3].reduce(map(square)(add));
// 14

notify = function(n) {console.log("Working on:", n); return n;};

[1,2,3].reduce(map(notify)(add), 0);
// Working on: 1
// Working on: 2
// Working on: 3
// 14