Map/Filter implemented using Fold

map_filter_using_fold.js

var filter = function (xs, p) {
  return xs.reduce(function (results, v) {
    return results.concat(p(v) ? [v] : []);
  }, []);
};

var map = function (xs, f) {
  return xs.reduce(function (results, v) {
    return results.concat([f(v)]);
  }, []);
};