installing Socket.IO on NodeJs server

I would use Socket.IO . I have read the official documentation and tried to do the same thing so I create my server :

// server.js

// BASE SETUP
// =============================================================================

// call the packages we need
var express = require("express"); // call express
var app = express();
var bodyParser = require("body-parser");

// define our app using express
var routerProj = require("./routes/ajoutProj");

var mongoose = require("mongoose");

mongoose.Promise = global.Promise;

mongoose.connect("mongodb://localhost:27017", {
  useMongoClient: true

  /* other options */
}); // connect to our database

mongoose.connection.on("error", function(error) {
  console.log("error", error);
});
app.use(function(req, res, next) {
  res.header("Access-Control-Allow-Origin", "*");
  res.header("Access-Control-Allow-Methods", "GET, POST, PUT ,DELETE");

  res.header(
    "Access-Control-Allow-Headers",
    "Origin, X-Requested-With, Content-Type, Accept"
  );
  next();
});
app.use(bodyParser.urlencoded({ extended: true }));
app.use(bodyParser.json());
app.use("/api/proj", routerProj);

app.get("/", function(req, res) {
  res.sendFile(__dirname + "../src/index.html");
});
// Chargement de socket.io

// Quand un client se connecte, on le note dans la console
io.sockets.on("connection", function(socket) {
  console.log("User is coonected!");
});
var port = process.env.PORT || 8081; // set our port

// START THE SERVER
// =============================================================================
var server = app.listen(port);
var io = require("socket.io").listen(server);

My angular index.htlm file is in this path relatively to server.js : ../src/app/index.html

When I restart server and angular app, then open new window I don't have a message on the servers's console telling me that a user is connected knowing that angular is making calls to the server api

I don't know where is the problem

Update

I have added socket.IO on client side

import { Injectable } from "@angular/core";
import { NouveauProjet } from "./models/nouveau-projet";
import { HttpClient, HttpResponse } from "@angular/common/http";
import { Observable } from "rxjs/Observable";
import "rxjs/add/operator/map";
import "rxjs/add/operator/catch";
import * as io from "socket.io-client";

@Injectable()
export class AjoutprojService {
  apiURL = "http://127.0.0.1:8080/api/proj/projets";
  private socket = io("http://localhost:8081");

  constructor(private http: HttpClient) {}

  getAllProj(): Observable<NouveauProjet[]> {
    return this.http.get<NouveauProjet[]>(
      "http://127.0.0.1:8081/api/proj/projets"
    );
  }
  getProj(id): Observable<NouveauProjet[]> {
    return this.http.get<NouveauProjet[]>(
      "http://127.0.0.1:8081/api/proj/nouvProjs/${id}"
    );
  }
  addProj(nouveauProjet: NouveauProjet): Observable<any> {
    return this.http.post<NouveauProjet[]>(
      "http://127.0.0.1:8081/api/proj/projets",
      nouveauProjet
    );
  }
}
/*  private handleError ( response: HttpResponse): Observable<any> {
    let errorMessage= `${response.status} - ${response.statusText}`;
    return Observable.throw(errorMessage);
  }*/

Restarted server , client , no result

Update 2 after adding socket.on('event', function(evt){ console.log(evt); });I get those errors :

Failed to load http://localhost:8081/socket.io/?EIO=3&transport=polling&t=M2tXQXh: The value of the 'Access-Control-Allow-Origin' header in the response must not be the wildcard '*' when the request's credentials mode is 'include'. Origin 'http://localhost:4200' is therefore not allowed access. The credentials mode of requests initiated by the XMLHttpRequest is controlled by the withCredentials attribute.

GET http://localhost:8081/socket.io/?EIO=3&transport=polling&t=M2tXQXh 404 (Not Found)

If I set res.header("Access-Control-Allow-Origin", "*"); To res.header("Access-Control-Allow-Origin", "http://localhost:4200"); I get this error

Failed to load http://localhost:8081/socket.io/?EIO=3&transport=polling&t=M2uichH: The value of the 'Access-Control-Allow-Credentials' header in the response is '' which must be 'true' when the request's credentials mode is 'include'. Origin 'http://localhost:4200' is therefore not allowed access. The credentials mode of requests initiated by the XMLHttpRequest is controlled by the withCredentials attribute.

I notice a difference in the error . Here The value of the 'Access-Control-Allow-Credentials' header in the response is '' When I put localhost:8081 : The value of the 'Access-Control-Allow-Credentials' header in the response is 'localhost:8081'