test.js

const ffmpeg = require('fluent-ffmpeg');

const c = new ffmpeg('video1.mp4')
  .addInput('video2.mp4')
  .on('error', function(err) {
    console.log('an error happened: ' + err.message);
  })
  .on('end', function() {
    console.log('final video saved');
  })      
  .mergeToFile('output.mp4', '/');

c.run();