Experimental extending of native objects without touching prototypes

natives.js

;(function(){

  function typeOf(item){
    if (item._family) return item._family();
    return typeof item;
  }

  Array.prototype._family = function(){ return 'array'; };

  var _ = this._ = function(obj){
    if (obj._extended) return obj;

    var wrapper = {
      _obj: obj,
      _type: typeOf(obj),
      _extended: true,
      get: function(){ return wrapper._obj }
    };

    for (i in exts[wrapper._type]){
      if (exts[wrapper._type].hasOwnProperty(i)){
        wrapper[i] = function(){
          return exts[wrapper._type][i].call(wrapper._obj, [].slice.call(arguments, 0));
        }
      }
    }

    return wrapper;
  };

  var exts = { array: {}, number: {}, string: {} };

  _.define = function(type, name, fn){
    exts[type][name] = fn;
  };

})();

Example usage: http://jsfiddle.net/rpflorence/DZEPZ/

_.define('array', 'sum', function(){
  var sum = 0;
  for (var i = this.length - 1; i >= 0; i--){
    sum += parseFloat(this[i]);
  };
  return _(sum);
});

_.define('number', 'round', function(precision){
  precision = Math.pow(10, precision || 0).toFixed(precision < 0 ? -precision : 0);
  return _(Math.round(this * precision) / precision);
});

var value = _([1, 1, 1.5]).sum().round().get(); // => 4