code: function (path) {
      let id;
  
      if (path === '/') {
        // "root" is a special alias for the fileId of the root folder
        return Promise.resolve('root');
      } else if ((id = this._fileIdCache.get(path))) {
        // id is cached.
        return Promise.resolve(id);
      }
      // id is not cached (or file doesn't exist).
      // load parent folder listing to propagate / update id cache.
      return this._getFolder(parentPath(path)).then(() => {
        id = this._fileIdCache.get(path);
        if (!id) {
          if (path.substr(-1) === '/') {
            return this._createFolder(path).then(() => {
              return this._getFileId(path);
            });
          } else {
            return Promise.resolve();
          }
        }
        return Promise.resolve(id);
      });
    }
docs: Get the ID of a file.
  
  @param {string} path - File path
  @returns {Promise} Resolves with the ID
  
  @private