code: function _processOnKeyUp(evt) {
          let key = evt.keyCode || evt.which;
          let char = String.fromCharCode(key);
          let self = this;
          if (key === 38) {
              if (self._idx > 0) {
                  _unselectWord(self._words[self._idx]);
                  self._idx--;
              }
              self._words[self._idx].className += ' ac__word--selected';
          } else if (key == 40) {
              if (self._idx < self._words.length - 1) {
                  _unselectWord(self._words[self._idx]);
                  self._idx++;
              }
              self._words[self._idx].className += ' ac__word--selected';
          } else if (key == 13) {
              if (self._words.length && self._idx > -1)
                  self._onSelectedCB(_getStringFromWordElement(self._words[self._idx]));
              else
                  self._onSelectedCB(self._input.value);
          } else if (/[a-zA-Z0-9-_ ]/.test(char) || key === 8) {
              self._idx = -1;
              let prefix = self._input.value;
              if (prefix != '')
                  self._keyListenerDebounced(prefix);
              else
                  _flushWordsContainer.call(self);
          }
      }
docs: Listen to a keyup event on the search input. This function will handle navigation
  inside the words ui widget by listening to up and down arrows.
  
  @param {event} evt - Keyup event