Nested callbacks

file.js

  addStepsAttachments(fileName, folder, callback) {
    fileName = '12345';
    fs.readdir(folder, 'utf8', function(err, filesList) {
      if (!err && filesList.length > 0) {
        const results = [];
        const matchingFiles = filesList
          .filter(file => file.startsWith(fileName))
          .map(file => path.resolve(folder, file));
        
        // just in case there were no matching files
        done()

        matchingFiles.forEach(file => {
          fs.readFile(filePath, {encoding: 'base64'}, function(err, data) {
            if (!err && data) {
              results.push(data);
              
              done()
            } else {
              callback(err);
            }
          });
        });
        
        // callback if all matching files were loaded
        function done(){ if(result.length === matchingFiles.length) callback(null, results)}
      } else {
        callback(`Dir ${dirPath} is empty or some error happened: ${err}`, []);
      }
    });
  }
    }