Issue while compiling down js and concatenating

Hey guys,

I am on Laravel 5.4 and desperately trying to compile down resource/assets/js/app.js and then concatenate it with who other js file. Whatever the Laravel Mix configuration, I can't seem to make it work properly. Here it is my code:

mix.js('resources/assets/js/app.js', 'public/js/app.js')
    .combine(['public/js/app.js',
        'resources/assets/js/selectr/selectr.js',
        'resources/assets/js/selectr/selectr-main.js'], 'public/js/app.js')
    .sass('resources/assets/sass/app.sass', 'public/css/app.css')
    .sourceMaps().version();

I can't compile all the files using the js() method because apparently the javascript code in selectr.js breaks. Can you please help?

Thanks.