(function (win, doc) {
  var by = {},
      slice = [].slice,
      type2method = {
        id: "getElementById",
        className: "getElementsByClassName",
        tagName: "getElementsByTagName",
        query: "querySelectorAll"
      };
    
  for (var type in type2method) {
    if (type2method.hasOwnProperty(type) && type2method[type] in doc) {
      (function (type, method) {
        by[type] = method.match(/(?:Elements|All)/) ? function () {
          return slice.call(doc[method].apply(doc, arguments));
        } : function () {
          return doc[method].apply(doc, arguments);
        };
      })(type, type2method[type]);
    }
  }

  win.by = by;
})(window, document)