I am setting up a server and every time I try running npm start I get the following error:

[nodemon] 2.0.12
[nodemon] to restart at any time, enter 
`rs`
[nodemon] watching path(s): *.*
[nodemon] watching extensions: js,mjs,json
[nodemon] starting `node index.js`      
[nodemon] clean exit - waiting for changes before restart

I have researched online such as StackOverflow, YouTube and even Reddit. However I do not understand why I am getting this error, since I added app.listen(). I have also ran npm updateto make sure that I have the most up to date npm version.

In addition, it seems that I need to revert to an older version of Nodemon, but I am wondering if there is a better way of doing this with more recent versions?

Here is what my code looks like:

index.js:

import express from "express";
import bodyParser from "body-parser";
import mongoose from "mongoose";
import cors from "cors";

const app = express();

app.use(express.json({ limit: "30mb", extended: true }));
app.use(express.urlencoded({ limit: "30mb", extended: true }));
app.use(cors());

// database connection string: PLEASE NOTE THAT I HAVE REPLACED MY PASSWORD WITH THE PROPER ONE HERE
const CONNECTION_URL =
  "mongodb+srv://admin2021:<password>@cluster0.haf0y.mongodb.net/myFirstDatabase?retryWrites=true&w=majority";
const PORT = process.env.PORT || 5000;

// connect to database
mongoose
  .connect(CONNECTION_URL, {
    useNewUrlParser: true,
    useUnifiedTopology: true,
  })
  .then(() =>
    app.listen(PORT, () => console.log(`Server running on port: ${PORT}`))
  )
  .catch(() => (error) => console.log(error.message));

package.json:

{
  "name": "server",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "type": "module",
  "scripts": {
    "start": "nodemon ./index.js"
  },
  // etc.
}

Please note that I tried replacing nodemon ./index.js with nodemon index.js but I obtain the same error.

Any help is appreciated.