Javascript, method chaining, how to realize response in callback?

Good afternoon, ladies and gentlemen. I write for myself a module on node js, I can not implement the save () function

module.exports = function(filename, callback){
    this.callback = callback(require(filename));
    return { 
        save: function() {
            console.log(this);
            //fs.writeFile(filename, 'Hello Node.js');
        }
    }; 
}

File('file.json', function(data){
    data['blablabla'] = 12345;
}).save();

How from a callback, to transfer the changed copy of a file in save (), and there already to save it through fs? I'm interested in the implementation of the fluent-ffmpeg function save ()

ffmpeg('/path/to/file.avi')
 .videoCodec('libx264')
 .audioCodec('libmp3lame')
 .size('320x240')
 .on('error', function(err) {
   console.log('An error occurred: ' + err.message);
 })
 .on('end', function() {
   console.log('Processing finished !');
 })
 .save('/path/to/output.mp4');

And as well as implemented on()?

on('end'),   
on('error'),   
on('response'),

I will be grateful for the information provided.

via CHBS