Generic Array subclassing function using ECMA-Harmony Proxy features

es-harmony_array_subclassing.js

// Generic function to subclass an array in JavaScript
// (using the ECMAScript Harmony Proxy features) 
// Structuring of code inspired by @kangax's
// array_subclassing (http://github.com/kangax/array_subclassing)

var makeSubArray = (function () {
  var MAX_UINT32 = Math.pow(2, 32) - 1;

  return function (properties) {
    properties || (properties = {});
    properties.length = { value: 0, writable: true };
    var obj = Object.create(Array.prototype, properties);

    function enumerate() {
      return Object.keys(obj);
    }

    var handler = {
      get: function (receiver, name) {
        return obj[name];
      },
      set: function (receiver, name, value) {
        var len, index = name >>> 0; // ToUint32

        if (index === +name && index < MAX_UINT32){
          len = index + 1;
          obj.length = Math.max(len, obj.length);
        } else if (name == 'length'){
          len = value >>> 0; // ToUint32

          if (len !== +value) {
            throw new RangeError();
          }

          for (var i = len; i < obj.length; i++) {
            delete obj[i];
          }
          value = len;
        }

        obj[name] = value;
      },
      hasOwn: function(name) {
        return Object.prototype.hasOwnProperty.call(obj, name);
      },
      has: function(name) {
        return name in obj;
      },
      enumerate: enumerate,
      enumerateOwn: enumerate,  // enumerateOwn presumably will be renamed to keys
      getOwnPropertyNames: function () { return Object.getOwnPropertyNames(obj); },
      getOwnPropertyDescriptor: function () { return Object.getOwnPropertyDescriptor(obj); }
    };
    return Proxy.create(handler, Array.prototype);
  };
})();

function SubArray(n) {
  var arr = makeSubArray();
  if (arguments.length == 1 && typeof n == 'number') {
    arr.length = arguments[0];
  } else {
    arr.push.apply(arr, arguments);
  }
  return arr;
}