dataset.js

(function () {
  var forEach = [].forEach,
      regex = /^data-(.+)/,
      dashChar = /\-([a-z])/ig,
      el = document.createElement('div'),
      mutationSupported = false,
      match
  ;

  function detectMutation() {
    mutationSupported = true;
    this.removeEventListener('DOMAttrModified', detectMutation, false);
  }

  function toCamelCase(s) {
    return s.replace(dashChar, function (m,l) { return l.toUpperCase(); });
  }

  function updateDataset() {
    var dataset = {};
    forEach.call(this.attributes, function(attr) {
      if (match = attr.name.match(regex))
        dataset[toCamelCase(match[1])] = attr.value;
    });
    return dataset;
  }

  // only add support if the browser doesn't support data-* natively
  if (el.dataset != undefined) return;

  el.addEventListener('DOMAttrModified', detectMutation, false);
  el.setAttribute('foo', 'bar');

  function defineElementGetter (obj, prop, getter) {
    if (Object.defineProperty) {
        Object.defineProperty(obj, prop,{
            get : getter
        });
    } else {
        obj.__defineGetter__(prop, getter);
    }
  }

  defineElementGetter(Element.prototype, 'dataset', mutationSupported
    ? function () {
      if (!this._datasetCache) {
        this._datasetCache = updateDataset.call(this);
      }
      return this._datasetCache;
    }
    : updateDataset
  );

  document.addEventListener('DOMAttrModified', function (event) {
    delete event.target._datasetCache;
  }, false);
})();