code: function existFile(filePath, mode = fs.constants.W_OK) {
    return new Promise((resolve, reject) => {
      fs.access(filePath, mode, (err) => {
        if (err) {
          reject(err);
          return;
        }
        resolve(filePath);
      });
    });
  }
docs: Check if a file exists
  @param {string} filePath path and filename: the file to control
  @returns {Promise<string>} resolve with the filePath that exists
  @throws {error} if the file doesn't exist