Generate bat file for concatenating MP3. Each 18 MP3 from directory is added 2 times into one biger MP3 file

concatenate-mp3.js

// Generate bat file for concatenating MP3. Each 18 MP3 from directory is added 2 times into one biger MP3 file
// Like this, but every line contains 2x18 mp3 files:
// ffmpeg -i "concat:50lang-1st-part-1-0001.mp3|50lang-1st-part-1-0002.mp3|50lang-1st-part-1-0003.mp3" -c copy 50lang-lesson-01.mp3

const path = require('path');
const fs = require('fs');

const directoryPath = path.join(__dirname);

let fff = [];

fs.readdir(directoryPath, function (err, files) {
    //handling error
    if (err) {
        return console.log('Unable to scan directory: ' + err);
    } 
    //listing all files using forEach
    files.forEach(function (file) {
        // Do whatever you want to do with the file
        fff.push(file); 
    });

    //console.log(fff.length);

    let lesson = 1;

    while (fff.length > 0) {

	    let cmd1 = "";
	    let set1 = [];

	    for (let i = 1; i < 19; i++) {

			set1.push(fff.shift());
		}

		set1 = set1.concat(set1); // double the content
		cmd1 = set1.join("|");

		let lessonStr = String(lesson);
		if (lesson < 10) {
			lessonStr = "0".concat(lessonStr);
		}

		console.log(`ffmpeg -i "concat:${cmd1}" -c copy 50lang-lesson-${lessonStr}.mp3`);
		lesson += 1;
    }

});