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How to get the socket id when parsing through io.sockets.sockets

I'm using Socket.io on a Node.js server.
for (connectedSocket of io.sockets.sockets) {
console.log(`TEST: id = ${connectedSocket.id}`) //Why is this 'undefined'? All I want is the unique identifier of each socket in the server.
}
Not sure how to fix this, but it has to be super simple. Sorry about the fail code formatting :o

As of socket.io v3, io.sockets is now a Map object as shown here. You can iterate it directly with:
for (let [id, socket] of io.sockets.entries()) {
// can use socket here
}
You can use the newer interface:
let socketsArray = await io.fetchSockets();
to get you an array of connected sockets.
Note, you can also use fetchSockets() with rooms and namespaces as shown here.

socket.io id is not conform and don't work

Whene I get id of socket (socket.id) he return "/privateroom#cL-qy2G6gVfwUY_ZAAAH" the socket is in "/privateroom" namespace. but whene I want to send a message to socket with id that doesn't work. in documentation the id is just "cL-qy2G6gVfwUY_ZAAAH" i don't understand why that is different.
my socket version is ^2.3.0

How to create sockets that particular to user and disposable on Socket.Io

I write a Node.Js app and I use Socket.Io as the data transfer system, so requests should be particular to per user. How can I make this?
My actual code;
node:
io.on('connection', (socket) => {
socket.on('loginP', data => {
console.log(data);
})
})
js:
var socket = io('',{forceNew : false});
$("#loginbutton").click(function() {
var sessionInfo = {
name : $("#login input[name='username']").val(),
pass : $("#login input[name='pass']").val()
}
socket.emit("loginP", sessionInfo)
})
It returns one more data for per request and this is a problem for me. Can I make this on Socket.Io or should I use another module, and If I should, which module?

If I understand your question correctly (It's possible I don't), you want to have just one connection from each user's browser to your nodejs program.
On the nodejs side, your io.on('connection'...) event fires with each new incoming user connection, and gives you the socket for that specific connection. So, keep track of your sockets; you'll have one socket per user.
On the browser side, you should build your code to ensure it only calls
var socket = io(path, ...);
once for each path (your path is ''). TheforceNew option is for situations where you have multiple paths from one program.

socket.io assigning custom socket.id

I keep track the list of every users connected in the array.
So if there is a new connection, it will check whether the user is already on the list or not, if he was already on the list, then assign their socket.id with the corresponding socket.id on the list, otherwise just add them to the list.
It's for preventing same user counted as 2 user while he attempt to do multi-login.
Object.keys(client).forEach(function (key) {
if (client[key].id == data.id){
is_connected = true;
socket.id = key;
}
});
I have no problem handling the messages/chat that was sent/received by the user who attempt multi-login.
socket.on('chat', function(msg){
var data = {"name": client[socket.id].name, "message": msg};
io.emit('chat', data);
});
The io.emit for the chat message was succesfully sent to the user who attempting multi-login.
The problem I got was whenever the user decide to logout/disconnect from the server.
io.emit('user_leave', client[socket.id].id);
[Multi-Login Case] -> Multi-User and Dual-User are same user attempting Multi-Login
Whenever the Main-User disconnected from the server, the Dual-User received 'user_leave' sent by the server, because io.emit supposed to send it to all sockets.
But not otherwise, while the Sub-User disconnected from the server, the Main-user do not receive 'user_leave' emitted by the server.
*Note: Main-User is login first, then the Dual-User. So the Main-User information was saved directly in the array, while the Sub-User socket.id was assigned with the Main-User socket.id
[Update]
B2 socket.id was assigned with B1 socket.id, the io.emit for chat work perfectly while io.emit for disconnect only emitted to All except Dual-User(B2)

socket.id is used internally by socket.io for its own socket list. You cannot overwrite that or you break some of its ability to maintain its own data structures.
You have two choices:
You can use the existing socket.id value as is (without overwriting it) so you don't break existing behavior. It is already guaranteed to be unique on the server.
You can use a different property name for your own id such as socket.userId and then you won't conflict.
If you need to, you can maintain a map between your own custom id and the socket.io socket.id so you could get to one from the other.
Similar question here: Socket.io custom client ID

generateId prop of io.engine object can be used for to set the custom id.
Using this way, the all socket ids can be created on the server side without any issue.
Actually I wrote an answer for a similar question today.
An example:
var app = require('express')();
var http = require('http').Server(app);
var io = require('socket.io')(http);
io.engine.generateId = function (req) {
// generate a new custom id here
return 1
}
io.on('connection', function (socket) {
// listing the default namespace rooms
console.log("rooms: ", io.nsps["/"].adapter.rooms);
})
The console output would be as the following:
rooms: { '/#1': Room { sockets: { '/#1': true }, length: 1 } }
It seems to be it has been handled.
It must be in mind that socket id must be unpredictable and unique value with considering security and the app operations!
Extra: If socket.id is returned as undefined because of your intense processes on your generateId method, async/await combination can be used to overcome this issue on node.js version 7.6.0 and later. handshake method of node_modules/engine.io/lib/server.js file should be changed as following:
former:
Server.prototype.handshake = function (transportName, req) {
var id = this.generateId(req);
...
}
new:
Server.prototype.handshake = async function (transportName, req) {
var id = await this.generateId(req);
...
}

Socket.IO socket.id changing for same client without reconnect

I'm making an application where I need to map sockets to some object. I thought I would be safe using the socket.id variable provided by Socket.IO. Unfortunately while debugging I found out that the socket.id changed without the client disconnecting/reconnecting or whatever.
Here's a small piece of code to show what I'm trying to do:
var io = require('socket.io').listen(8080),
var myVariable = new Array();
// Main event loop
io.sockets.on('connection', function (socket) {
socket.on('myEvent', function(data) {
myVariable[socket.id] = data;
}
// 'someOtherEvent' which uses the mapped data.
socket.on('someOtherEvent', function(data) {
// Doesn't always work because socket.id has changed and var is empty
doSomethingWith(myVariable[socket.id]);
}
});
I'm not sure but I don't think this is the desired effect. Why does this happen and how would I work around it?

I was using node-inspector to debug my application and because I was holding the code on some breakpoint it disconnected the client (probably some timeout client side). That's why I got a new socket.id when I continued the code execution.

Looks like 'someOtherEvent' emited before 'myEvent' or socket has been reconnected. I use socket.id as connection identifier on two different projects in production and it works without any problems.