code: function () {
              if (this._cache.userAgent === undefined) {
                  this._cache.userAgent = impl.findMatch(impl.mobileDetectRules.uas, this.ua);
              }
              return this._cache.userAgent;
          }
docs: Returns the (first) detected user-agent string or <tt>null</tt>.
  <br>
  The returned user-agent is one of following keys:<br>
  <br><tt>{{{keys.uas}}}</tt><br>
  <br>
  In most cases calling {@link MobileDetect#userAgent} will be sufficient. But there are rare
  cases where a mobile device pretends to be more than one particular browser. You can get the
  list of all matches with {@link MobileDetect#userAgents} or check for a particular value by
  providing one of the defined keys as first argument to {@link MobileDetect#is}.
  
  @returns {String} the key for the detected user-agent or <tt>null</tt>
  @function MobileDetect#userAgent