Why Heroku app crashes following H10 and how to spot the problem? - node.js

Question

I am deploying an app to node.js mongodb-Atlas hosted Heroku app and after deploying it, it started to crash. here is the log of the incident:
2019-11-07T14:03:44.54763+00:00 app[web.1]:
2019-11-07T14:03:44.547905+00:00 app[web.1]: npm ERR! A complete log of this run can be found in:
2019-11-07T14:03:44.548078+00:00 app[web.1]: npm ERR! /app/.npm/_logs/2019-11-07T14_03_43_920Z-debug.log
2019-11-07T14:03:44.609666+00:00 heroku[web.1]: Process exited with status 1
2019-11-07T14:03:44.660116+00:00 heroku[web.1]: State changed from starting to crashed
2019-11-07T14:08:46.771752+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/" host=flix-app-test.herokuapp.com request_id=14978e4f-fb93-44c8-a6c1-294a6a254e1e fwd="84.17.61.226" dyno= connect= service= status=503 bytes= protocol=https
2019-11-07T14:08:47.381666+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/favicon.ico" host=flix-app-test.herokuapp.com request_id=b067c4c5-8b09-44c7-907e-bb2010dd97ec fwd="84.17.61.226" dyno= connect= service= status=503 bytes= protocol=https
2019-11-07T14:15:51.61258+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/" host=flix-app-test.herokuapp.com request_id=5290a4ea-94b1-4d79-a8fe-6709e6cb2a15 fwd="91.168.132.252" dyno= connect= service= status=503 bytes= protocol=https
2019-11-07T14:15:52.254787+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/favicon.ico" host=flix-app-test.herokuapp.com request_id=41f76767-00c8-490c-9c13-22c0906fedb9 fwd="91.168.132.252" dyno= connect= service= status=503 bytes= protocol=https
Most posts I found regarding this H10 error point to an issue on the lines that define the port which through Heroku listens, that it does not accept a rigid one, but the listening of the app is done by this code:
var port = process.env.PORT || 3000;
app.listen(port, "0.0.0.0", function() {
console.log('Listening on Heroku defined port');
});
The code is working locally (double checked), but the Heroku app does not even loads. Databases are running and available and I could find no visible problem.
I'd like to know how should I proceed to find out which part of the code is broken OR to find out that it's a Heroku temporarily issue.
Thanks in advance.

Answer 1

If you are using ExpressJS, just do:
var port = process.env.PORT;
Promise.resolve(app.listen(port)).then(() => {
console.log("Running!");
});
Anyways, 503 is Service Unavailable HTTP code. If the above code does not work, please post your router and controller code for the errored routes.