code: function () {
              if (this._cache.os === undefined) {
                  this._cache.os = impl.detectOS(this.ua);
              }
              return this._cache.os;
          }
docs: Returns the detected operating system string or <tt>null</tt>.
  <br>
  The operating system is one of following keys:<br>
  <br><tt>{{{keys.oss}}}</tt><br>
  
  @returns {String} the key for the detected operating system.
  @function MobileDetect#os