by.js

(function (win, doc) {
  var by = {}, slice = [].slice, type2method = {
    id: "getElementById",
    className: "getElementsByClassName",
    tagName: "getElementsByTagName",
    query: "querySelectorAll"
  };

  for (var type in type2method) if (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)