Probably is a port definition problem. Heroku will assign a port on which the application will run, regardless of you define it. You should replace that hardcoded port for something like this.
var port = process.env.PORT || 5000;
Hopefully, that fix the problem

Probably is a port definition problem. Heroku will assign a port on which the application will run, regardless of you defining it. You should replace that hardcoded port for something like this.
var port = process.env.PORT || 5000;
Hopefully that fix the problem