How to debug this heroku & nodejs express error? - node.js

Question

I'm trying to deploy my first node.js app on Heroku. However I get an error.
I looked everywhere and can't find a solution for this problem. Can someone please help me??
This is my app.js code:
const express = require('express')
const app = express()
const port = process.env.PORT || 5000
app.get('/', (req, res) => {
res.send('Hello World!')
})
app.listen(port, () => {
console.log(`Example app listening on port ${port}`)
})
These are the Heroku logs:
heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/" host=myapp.herokuapp.com request_id=3cbb2434-964b-4aed-9872-4db77650d7b2 fwd="93.172.35.247" dyno=
connect= service= status=503 bytes= protocol=https
app[api]: Build started by user myemail#gmail.com
app[api]: Build succeeded
app[api]: Deploy 9bb9c0e0 by user myemail#gmail.com
app[api]: Release v10 created by user myemail#gmail.com
heroku[web.1]: State changed from crashed to starting
heroku[web.1]: Starting process with command `npm start`
app[web.1]: > be#1.0.0 start /app
app[web.1]: > node app.js
app[web.1]: App is running, server is listening on port undefined
heroku[web.1]: Error R10 (Boot timeout) -> Web process failed to bind to $PORT within 60 seconds of launch
heroku[web.1]: Stopping process with SIGKILL
heroku[web.1]: Process exited with status 137
heroku[web.1]: State changed from starting to crashed