node js, ReferenceError: app is not defined - node.js

Question

1.here is my websocket.service.ts file
import { Injectable } from '#angular/core';
import { Observable, } from 'rxjs';
import {io} from 'socket.io-client';
#Injectable({
providedIn: 'root'
})
export class WebsocketService {
socket: any;
readonly uri: string = "ws://localhost:3000";
constructor() {
this.socket = io(this.uri);
}
listen(eventName: string, data:any){
return new Observable((subscriber) => {
this.socket.on(eventName, (data) => {
subscriber.next(data)
})
})
}
emit(eventName: string , data: any){
this.socket.emit(eventName,data);
}
}
here is my app.components.ts file
import { Component, OnInit } from '#angular/core';
import { WebsocketService } from './websocket.service';
#Component({
selector: 'app-root',
templateUrl: './app.component.html',
styleUrls: ['./app.component.css']
})
export class AppComponent implements OnInit {
title = 'socket-io-example';
constructor( private websockerservice: WebsocketService){
}
ngOnInit(){
// here we want to listen to an event from socket.io server
this.websockerservice.listen('test event','data').subscribe((data) =>{
console.log(data);
})
}
}
here is my node.js, socket-io.server.js file
var server = require('http').Server(app); //here i got refernce error: app is not defined
~~~
var io = require('socket.io')(server);
io.on('connection', function(socket){
console.log("A user cconnected");
socket.emit('test event', 'here is some data');
});
server.listen(3000, () => {
console.log("server.io server is listing to port 3000")
});

Answer 1

You must define app first. With express you can do the following:
const express = require('express');
var app = express();