exists(key) {
        if (!key) throw new Error('localStorageService - exists() - key is required');

        var exists = null;

        try {
          exists = this.get(key) !== null && this.get(key) !== undefined;
        } catch (e) {
          exists = CACHE[key] ? true : false;
        }

        return exists;
      }
@desc check if a value exists by key
@param {String} key
@return {Boolean}