server.js

const app = require("express")();
const http = require("http");
const { Server } = require("socket.io");
const cors = require("cors");

app.use(cors());

const server = http.createServer(app);

const io = new Server(server, {
  cors: {
    origin: "*",
    methods: ["GET", "POST"],
  },
});

app.get("/", function (req, res) {
  res.send("Hello from the server!");
});

const socketID_to_Users_Map = {};
const roomID_to_Code_Map = {};

async function getUsersinRoom(roomId, io) {
  const socketList = await io.in(roomId).allSockets();
  const userslist = [];
  socketList.forEach((each) => {
    each in socketID_to_Users_Map &&
      userslist.push(socketID_to_Users_Map[each].username);
  });

  return userslist;
}

async function updateUserslistAndCodeMap(io, socket, roomId) {
  socket.in(roomId).emit("member left", {
    username: socketID_to_Users_Map[socket.id].username,
  });

  // update the user list
  delete socketID_to_Users_Map[socket.id];
  const userslist = await getUsersinRoom(roomId, io);
  socket.in(roomId).emit("updating client list", { userslist: userslist });

  userslist.length === 0 && delete roomID_to_Code_Map[roomId];
}

//Whenever someone connects this gets executed
io.on("connection", function (socket) {
  console.log("A user connected", socket.id);

  socket.on("when a user joins", async ({ roomId, username }) => {
    console.log("username: ", username);
    socketID_to_Users_Map[socket.id] = { username };
    socket.join(roomId);

    const userslist = await getUsersinRoom(roomId, io);

    // for other users, updating the client list
    socket.in(roomId).emit("updating client list", { userslist: userslist });

    // for this user, updating the client list
    io.to(socket.id).emit("updating client list", { userslist: userslist });

    // send the latest code changes to this user when joined to existing room
    if (roomId in roomID_to_Code_Map) {
      io.to(socket.id).emit("on language change", {
        languageUsed: roomID_to_Code_Map[roomId].languageUsed,
      });
      io.to(socket.id).emit("on code change", {
        code: roomID_to_Code_Map[roomId].code,
      });
    }

    // alerting other users in room that new user joined
    socket.in(roomId).emit("new member joined", {
      username,
    });
  });

  // for other users in room to view the changes
  socket.on("update language", ({ roomId, languageUsed }) => {
    if (roomId in roomID_to_Code_Map) {
      roomID_to_Code_Map[roomId]["languageUsed"] = languageUsed;
    } else {
      roomID_to_Code_Map[roomId] = { languageUsed };
    }
  });

  // for user editing the code to reflect on his/her screen
  socket.on("syncing the language", ({ roomId }) => {
    if (roomId in roomID_to_Code_Map) {
      socket.in(roomId).emit("on language change", {
        languageUsed: roomID_to_Code_Map[roomId].languageUsed,
      });
    }
  });

  // for other users in room to view the changes
  socket.on("update code", ({ roomId, code }) => {
    if (roomId in roomID_to_Code_Map) {
      roomID_to_Code_Map[roomId]["code"] = code;
    } else {
      roomID_to_Code_Map[roomId] = { code };
    }
  });

  // for user editing the code to reflect on his/her screen
  socket.on("syncing the code", ({ roomId }) => {
    if (roomId in roomID_to_Code_Map) {
      socket
        .in(roomId)
        .emit("on code change", { code: roomID_to_Code_Map[roomId].code });
    }
  });

  socket.on("leave room", ({ roomId }) => {
    socket.leave(roomId);
    updateUserslistAndCodeMap(io, socket, roomId);
  });

  socket.on("disconnecting", (reason) => {
    socket.rooms.forEach((eachRoom) => {
      if (eachRoom in roomID_to_Code_Map) {
        updateUserslistAndCodeMap(io, socket, eachRoom);
      }
    });
  });

  //Whenever someone disconnects this piece of code executed
  socket.on("disconnect", function () {
    console.log("A user disconnected");
  });
});

//you can store your port number in a dotenv file, fetch it from there and store it in PORT
//we have hard coded the port number here just for convenience
const PORT = process.env.PORT || 5000;

server.listen(PORT, function () {
  console.log(`listening on port : ${PORT}`);
});