server-side script of my real time chat

const io = require("socket.io")(3000, { cors: { origin: "*", }, });
var fs = require('fs');

io.on("connection", socket => {
    console.log("new User")
    socket.emit("sent_message", "Hello World");

    socket.on("send_message", message => {
        socket.broadcast.emit("sent_message", message);
    });

    socket.on("username", data => {
        console.log("username");
        // data = JSON.stringify(users_data);
        // fs.writeFile('users.json', data, function (err) {
        //     if (err) throw err;
        //     console.log('Saved!');
        // });
        saved_data = fs.readFileSync('users.json', 'utf8' , (err) => {
            if (err) {
              console.error(err)
              return
            }
        });
        users = JSON.parse(saved_data);
        exists=false;
        saved_index=0;
        for(let i =0; i < users.length;i++){
            if(users[i].username==data){
                exists=true;
                saved_index = i;
            }
        }
        if(exists==false){
            users.push({username:data, password:""});
            save_data = JSON.stringify(users);
            fs.writeFile('users.json', save_data, function (err) {
                    if (err) throw err;
                    console.log('Saved!');
            });
            console.log("sending pass");
            socket.emit("create_password");
        }else{
            if(users[saved_index].password==""){
                socket.emit("create_password");
                console.log("create_password");
            }else{
                socket.emit("login_password");
                console.log("login_password");
                console.log(":"+users[saved_index].username);
                console.log(":"+users[saved_index].password);

                console.log(":"+users[saved_index].password);
            }
        }


    });

    socket.on("save_password", recieved_data => {
        console.log("got");
        saved_data = fs.readFileSync('users.json', 'utf8' , (err) => {
            if (err) {
              console.error(err)
            }
        });
        users = JSON.parse(saved_data);
        console.log("recived: "+recieved_data[0]+"  and  "+recieved_data[1]);
        for(let i=0;i<users.length;i++){
            if(users[i].username == recieved_data[0]){
                users[i].password = recieved_data[1];
                console.log("saved as"+ recieved_data[0]+" and "+recieved_data[1]);
            }
        }
        save_data = JSON.stringify(users);
        fs.writeFile('users.json', save_data, function (err) {
            if (err) throw err;
            console.log('Saved!');
        });

        socket.emit("end_login");
    });

    socket.on("check_password", recieved_data =>{
        saved_data = fs.readFileSync('users.json', 'utf8' , (err) => {
            if (err) {
              console.error(err)
              return
            }
        });
        users = JSON.parse(saved_data);

        for(let i=0;i<users.length;i++){
            if(users[i].username == recieved_data[0]){
                if(users[i].password==recieved_data[1]){
                    socket.emit("correct_password");
                }
                else{
                    console.log("bout to send inccorect pass");
                    socket.emit("incorrect_password");
                }
            }
        }
    });

    socket.on("logged_in", recieved_data =>{
        socket.broadcast.emit("new_partner", recieved_data);
    });
})