code: function (entry) {
        // update most number to key
        if (entry.index !== this._head - 1) {
          var isTail = entry.index === this._tail;
          delete this._byOrder[entry.index];
          entry.index = this._head++;
          this._byOrder[entry.index] = entry;
          if (isTail) this._shift();
        }
      }
docs: update least recently used index of an entry to "_head"
  
  @param {Entry}
  @api private