Pretending to be an Array (like DOMTokenList needs to do)

gistfile1.js

function DOMTokenList() {
  Object.defineProperty(this, 'length', {
    enumerable: true,
    configurable: false,
    get: function() {
      var tLargestNumber = -1;
      var tKeys = Object.keys(this);
      var tTemp, tKey;

      for (var i = 0, il = tKeys.length; i < il; i++) {
        tKey = tKeys[i];
        tTemp = parseInt(tKey, 10);
        if ((tTemp + '') === tKey) {
          if (tTemp > tLargestNumber) {
            tLargestNumber = tTemp;
          }
        }
      }

      return tLargestNumber + 1;
    }
  });
}

DOMTokenList.prototype.item = function(pIndex) {
  return this[pIndex + ''];
}