How do i define 'db' i trying connecting to connect mongodb to nodejs

I think you have problem with your file structure. Probably you are not including db to your controller.

Please, create a new file and call it database.js

Design your database.js like this:

const { MongoClient } = require('mongodb');

class DatabaseSingleton {

    constructor() {
        const uri = process.env.DB_CONNECTION_STRING;
        
        this.client = new MongoClient(uri, { useUnifiedTopology: true });
        this.isConnected = false;

        this.client.connect().then(()=>{this.isConnected = true});
    }

    /**
     * Returns the promise of a Database object. (It waits until a connection to the database was successful)
     * @returns {Promise<any>}
     */
    get mongo() {
        return new Promise(async (resolve, reject) => {
            let timedOut = false;
            setTimeout(()=>{timedOut = true}, 300 * 1000);
            while (!this.isConnected && !timedOut){
                await new Promise((resolve)=>{setTimeout(()=>{resolve()}, 100)}) // Snooze the thread for 100ms without blocking the event loop
            }

            if (timedOut){
                reject(false);
            } else {
                resolve(this.client.db());
            }
        })
    }
}

// *** Setting up singleton ***
const SINGLETON_KEY = Symbol.for("MyProject.MyDB");

var globalSymbols = Object.getOwnPropertySymbols(global);
var hasInstance = (globalSymbols.indexOf(SINGLETON_KEY) > -1);

// If there is no instance of the singleton, make the instance.
if (!hasInstance){
    global[SINGLETON_KEY] = new DatabaseSingleton()
}

// Define a function to retrieve a singleton instance
var singleton = {};
Object.defineProperty(singleton, "instance", {
    get: function(){
        return global[SINGLETON_KEY];
    }
});

// The singleton should never be changed
Object.freeze(singleton);

module.exports = singleton;

// *** End of singleton setup ***

And convert your controller something like:

const database = require('./database').instance;

 
 app.post('/create-item', function(req, res) {
  const db = await database.mongo; // Call db here
  db.collection('item').insertOne({text: req.body.item}, function() {
  res.send("Thanks for submitting the form.")
  })

Try this. Create a db.js file to setup database connection only and export the client.

    const dotenv = require("dotenv");
dotenv.config();
const mongodb = require("mongodb");
mongodb.connect(
    process.env.CONNECTIONSTRING,
    {
        useNewUrlParser: true,
        useUnifiedTopology: true,
    },
    (err, client) => {
        module.exports = client
        const server = require("./server");
        server.listen(process.env.PORT);
    }
);

Now in your server.js file require the client that are exported.

const client = require('./client')

now use this as

client.db('databaseName').collection('item').insertOne({})