functional.js

// usage:

// var double = function(a){Functional.map(a, function{return 2*a})};
// double([1,2,3]) -> [2,4,6]

// var even = function(a){Functional.map(a, function{return a%2 == 0})};
// even([1,2,3,4,5,6,7]) -> [2,4,6]


var Functional = {
  isFunction: function(obj) {
    return toString.call(obj) === "[object Function]";
  },

  reduce: function(accumulator, object, callback) {
    var name, i = 0,
            length = object.length,
            isObj = length === undefined || Functional.isFunction(object);

    var acc = accumulator;

    if (isObj) {
      for (name in object) {
        acc = callback.call(object[ name ], acc, object[ name ], name);
      }
    } else {
      for (var value = object[0]; i < length; value = object[++i]) {
        acc = callback.call(value, acc, value, i);
      }
    }

    return acc;
  },

  map: function(object, callback) {
    return Functional.reduce([], object, function(acc, index_key, value) {
      var m = callback.call(value, value, index_key);
      acc.push(m);
      return acc;
    });
  },

  filter: function(object, callback) {
    var isObj = object.length === undefined || Functional.isFunction(object);
    if (isObj) {
      return Functional.reduce({}, object, function(acc, value, index_key) {
        if(callback.call(value, value, index_key) !== false){
          acc[index_key] = value;
        }
        acc.prototype = object.prototype;
        return acc;
      });
    } else {
      return Functional.reduce([], object, function(acc, value, index_key) {
        if(callback.call(value, value, index_key) !== false){
          acc.push(value);
        }
        return acc;
      });
    }

  }
};