Using FFMPEG to concatenate files on Google Cloud Functions via NodeJS .. send it array of filenames and the extension type and it returns a promise that resolves to the final file with full path

ffmpeg_concatenate.js

const os = require("os");
const ffmpegPath = require("@ffmpeg-installer/ffmpeg").path;
const fluent_ffmpeg = require("fluent-ffmpeg");
const ffprobePath = require('@ffprobe-installer/ffprobe').path;
const path = require('path');

fluent_ffmpeg.setFfprobePath(ffprobePath); //this was key for me on GCF
fluent_ffmpeg.setFfmpegPath(ffmpegPath);

/**
 * 
 * @param {Array} filearr array of file names with path 
 * @param {String} fileType file extension 
 */
function concatenate(filearr, fileType) {
    const tmpdir = os.tmpdir();

    let mpeg = new fluent_ffmpeg(); 
    let outFileName = path.join(tmpdir, 'finalFile.' + fileType);

    filearr.forEach(function (fileName) {
        mpeg.addInput(fileName);
    });
    
    const promise = new Promise((resolve, reject) => {
        mpeg.mergeToFile(outFileName, tmpdir)
            .on('error', function (err, stdout, stderr) {
                console.log(err.message); 
                console.log("stdout:\n" + stdout);
                console.log("stderr:\n" + stderr); 
                reject;
            })
            .on('end', function () {
                console.log('Finished concatenating!');
                resolve(outFileName);
            });
    });
    return promise;
}
exports.concatenate = concatenate;