server.js

let express = require("express");
let socket = require("socket.io");
let MongoClient = require('mongodb').MongoClient;
let url = "mongodb://localhost:27017/chat";

const client = new MongoClient(url, { useUnifiedTopology: true });


MongoClient.connect(url, { useUnifiedTopology: true },function(err, db) {   //here db is the client obj
    if (err) throw err;

    db.auth('chat_admin', 'chat_password')
    let dbase = db.db("chat"); //here

    let allCollections = [];

    dbase.listCollections().toArray(function(err, collections) {
        if(err) console.log(err);
        //iterate to each collection detail and push just name in array
        collections.forEach(eachCollectionDetails => {
            allCollections.push(eachCollectionDetails.name);
        });
        //close client
        // client.close();
    });

    console.log(allCollections);

    // if (!allCollections.includes("chats")) {
    //
    //     dbase.createCollection("chats", function (err, res) {
    //         if (err) throw err;
    //         console.log("Chats database created!");
    //         db.close();   //close method has also been moved to client obj
    //     });
    // }else{
    //     console.log("Chats database found.");
    // }
});

let app = express();

app.use(express.static("public"));

let server = app.listen(3456, function() {
    console.log("Listening to port 3456.");
});

let io = socket(server);

// Global variables to hold all usernames and rooms created
let usernames = {};
let rooms = ["global", "chess", "video-games"];

io.on("connection", function(socket) {

    console.log("User connected to server.");

    socket.on("createUser", function(username) {
        socket.username = username;
        usernames[username] = username;
        socket.currentRoom = "global";
        socket.join("global");
        socket.emit("updateChat", "INFO", "You have joined global room");
        socket.broadcast
            .to("global")
            .emit("updateChat", "INFO", username + " has joined global room");
        io.sockets.emit("updateUsers", usernames);
        socket.emit("updateRooms", rooms, "global");
    });


    socket.on("sendMessage", function(data) {
        io.sockets
            .to(socket.currentRoom)
            .emit("updateChat", socket.username, data);

        db.collection('chat').insertOne(doc, function (error, response) {
            if(error) {
                console.log('Error occurred while inserting');
                // return
            } else {
                console.log('inserted record', response.ops[0]);
                // return
            }
        });
    });


    socket.on("createRoom", function(room) {
        if (room != null) {
            rooms.push(room);
            io.sockets.emit("updateRooms", rooms, null);

        }
    });


    socket.on("updateRooms", function(room) {
        socket.broadcast
            .to(socket.currentRoom)
            .emit("updateChat", "INFO", socket.username + " left room");
        socket.leave(socket.currentRoom);
        socket.currentRoom = room;
        socket.join(room);
        socket.emit("updateChat", "INFO", "You have joined " + room + " room");
        socket.broadcast
            .to(room)
            .emit("updateChat", "INFO", socket.username + " has joined " + room + " room");
    });


    socket.on("disconnect", function() {
        delete usernames[socket.username];
        io.sockets.emit("updateUsers", usernames);
        socket.broadcast.emit("updateChat", "INFO", socket.username + " has disconnected");
    });

});