node.js do not work after deploy on Heroku - node.js

Question

I am new to node.js and Heroku, I just have deployed for the first time a node.js app to Heroku and when I ran in Heroku, the application did not run so I used the command heroku logs --tail but I had this error:
enter image description here
For idea, my node.js works very fine without Heroku but stop working after deploying it there
Here my index.js:
const express = require("express");
var md5 = require('md5');
var reverseMd5 = require('reverse-md5');
const app = express();
const userRouter = require("./API/users/user.router");
const operationsRouter = require("./API/operations/operations.router");
app.use(express.json());
app.use("", userRouter);
app.use("", operationsRouter);
app.use(express.static('images'));
const port = process.env.APP_PORT || 3000;
app.listen(port, () => {
console.log("server up and running on PORT :", port);
});
Hero my config file java:
const { createPool } = require("mysql");
const pool = createPool({
host: 'host.example.com',
port: 3306,
user: 'user',
password: 'password',
database: 'database',
connectionLimit: 100,
multipleStatements: true
});
module.exports = pool;
What shocked me is that why these errors show up only after deploying on Heroku?
Is there anything I can do to fix this problem?

Answer 1

Heroku dynamically assigns your app a port, so you can't set the port to a fixed number. Heroku adds the port to the env, so you can pull it from there. Switch your APP_PORT listen to this:
.listen(process.env.PORT || 5000)
Make sure you have changed the APP_PORT to PORT.
That way it'll still listen to port 5000 when you test locally, but it will also work on Heroku.
One more thing I can see that you are using Nodemon on Heroku which is not necessary on Heroku. So remove the Nodemon from your Heroku and declare a Procfile
web: node app.js
Being app.js the entry point to your app.