Rename all files in a directory

renameFiles.js

// Copy into directory you want to rename.
const fs = require('fs');

const BASE_NAME = 'BaseFile_Name';
const FILE_EXT = 'm4b';

fs.readdir('./', (err, files) => {
  // Filter files by filetype, so we can ignore rename function.
  const filteredFiles = files.filter((file, i) => {
    return file.indexOf(FILE_EXT) > -1;
  });

  console.log(filteredFiles);

  filteredFiles.forEach((file, i) => {
    let formatNumber = i + 1;

    if (formatNumber < 10) {
      formatNumber = '0' + formatNumber;
    }

    fs.rename('./' + file, './' + BASE_NAME + formatNumber + '.m4b', function (
      err
    ) {
      if (err) console.log('ERROR: ' + err);
    });
  });
});