code: function calculateSize(filepath) {
      return new Promise((fulfill, reject) => {
          fs.stat(filepath, (err, stats) => {
              if (err) return reject(err);
              fulfill(stats.size);
          });
      });
  }
docs: Calculates size of the file at the given path.
  
  @param {string} filepath
  @returns Promise<number> hex encoded hash