JAVASCRIPT

mongoparseerror: option usecreateindex is not supported

This happend because in mongoose version 6 and above don't require those
" { useNewUrlParser: true, 
     useUnifiedTopology: true, 
     useCreateIndex: true, 
     useFindAndModify: false,} "
so just delet it.
And if you still see "app crashed - waiting for file changes before starting..."
just save it one more time and it will work

MongoParseError: option usecreateindex is not supported

const express = require('express');
const app = express();
const mongoose = require('mongoose');

require('dotenv').config()

const connection_string = process.env.CONNECTION_STRING
//const port = process.env.PORT || 80

app.get('/', (req, res) => {
    res.send("Welcome to our ToDo")
})

app.listen(80,() =>{
    console.log("Server running on port 80.")
})

mongoose.connect(connection_string, {
    useNewUrlParser: true,
    useCreateIndex: true,
    useUnifiedTopology: true

})
.then(() => console.log('MongoDB connection established.'))
.catch((error) => console.error("MongoDB connection failed:", error.message))