TypeError: Cannot read property 'db' of undefined while trying to Mongodb Atlas Online

Question

[nodemon] starting node server.js C:\Users\Abhay\Desktop\todo-app\node_modules\mongodb\lib\utils.js:725 throw error; ^

TypeError: Cannot read property 'db' of undefined at C:\Users\Abhay\Desktop\todo-app\server.js:8:17 at C:\Users\Abhay\Desktop\todo-app\node_modules\mongodb\lib\utils.js:722:9 at C:\Users\Abhay\Desktop\todo-app\node_modules\mongodb\lib\mongo_client.js:223:23 at C:\Users\Abhay\Desktop\todo-app\node_modules\mongodb\lib\operations\connect.js:279:21 at QueryReqWrap.callback (C:\Users\Abhay\Desktop\todo-app\node_modules\mongodb\lib\core\uri_parser.js:56:21) at QueryReqWrap.onresolve [as oncomplete] (dns.js:202:10) [nodemon] app crashed - waiting for file changes before starting...

let express = require('express')
let mongodb = require('mongodb')
let app = express()
let db

let connectionString = 'mongodb+srv://todoAppUser:[email protected]/TodoApp?retryWrites=true&w=majority'
mongodb.connect(connectionString, {useNewUrlParser: true, useUnifiedTopology: true}, function(err, client) {
    db = client.db()
    app.listen(3000)
})

Answer 1

It seems you're trying to use the static connect method of MongoClient to make a connection to your db, but you are not using the MongoClient class itself.

To connect to any db, you will need a connected instance of MongoClient. Using the static connect method, you can achieve it in the following way:

const mongodb = require("mongodb");


const connectionURL = "mongodb+srv://your-connection-srv-here"
const dbName = "your_db_name"

//get MongoClient
const MongoClient = mongodb.MongoClient;

let db = null;

MongoClient.connect(connectionURL,{
    useNewUrlParser: true,
    useUnifiedTopology: true
},(err,connectedClient) => {
    if(err){
        throw err;
    }
    //connectedClient will be the connected instance of MongoClient
    db = connectedClient.db(dbName);
    //now you can write queries

    db.collection("your_collection").find({}).toArray()
    .then(r => {
        console.log(r);
    }).catch(e => {
        console.error(`ERROR:`,e);
    })

})

However, using callbacks will be quite cumbersome. As per the docs linked above, most functions in the MongoDb driver for Node.js will return a promise if a callback function is not passed, which is very convenient. Using this, you can write a function which return a promise that resolves a connected instance to your db.

const MongoClient = require('mongodb').MongoClient;


/*
    we draw the connection srv and the db name from the config to return just one instance of that db.
    Now this function call be called wherever a connection is needed
*/
const getDbInstance = (config) => new Promise((resolve,reject) => {
    const client = new MongoClient(config.dbUrl, {
        useNewUrlParser: true,
        useUnifiedTopology: true
    });
    client.connect((error) => {
        if(error){
            console.error(error);
            reject(error);
        }
        let db = client.db(config.dbName);
        resolve(db);
    })
})


const doSomeDbOperations = async() => {
    //hardcoding it here, but this config will probably come from environment variables in your project
    const config = {
        dbUrl: "mongodb+srv://your-connection-srv-here",
        dbName: "your_db_name"
    };

    try{
        const db = await getDbInstance(config);

        //do whatever querying you wish here

    }catch(e){
        console.error(`ERROR: `,e);
    }

}

doSomeDbOperations();