How will Node.JS process loop control while there's async function called in the loop?

Question

How will Node.JS process loop control while there's async function called in the loop?

My situation is as follows: there's an array of IP address. I will test each IP to connect to a remote server. If one IP connects, the rest IPs are ignored and not going to be connected.

I used the following Node.JS codes to do the work, but it seems not working. Please give some hints. Thanks!

// serverip is a var of string splitted by ";", e.g. "ip1;ip2;ip3"
var aryServerIP = serverip.split(";");
console.log(aryServerIP);

var ipcnt = aryServerIP.length; // ipcnt = 3, for example
for (ip in aryServerIP)
{
    console.log("to process: " + ipcnt); // error here: always print 3
    var net = require('net');
    var client = new net.Socket();
    var rdpport = 3389;
    client.connect(rdpport, aryServerIP[ip], function(){
        console.log("socket connected to " + aryServerIP[ip] + ":" + rdpport);
        client.destroy();
        if (0 != ipcnt)
        {
            // do some real connection work about aryServerIP[ip].
            ipcnt--;
        }
    });
    client.on('error', function(){
        console.log("fail to connect to " + aryServerIP[ip] + ":" + rdpport);
        ipcnt--;
    });
}

I know using ipcnt count to control the loop is bad, but I don't know how to control the Node.JS loop, when there's async function called in the loop...

node.js
loops
asynchronous

Answer 1

Because your connect and error callbacks are both asynchronous, so they will both run after the for loop has completely finished.

What you need to do is set up a set of callbacks. For instance, rather than use a for loop, wrap your entire loop body in a function. If connect succeeds, then just do what you normally would, and if the error callback is called, then execute the wrapping function again. Something like this:

var async = require('async');
var net = require('net');
var rdpport = 3389;

function tryConnections(aryServerIP, callback){
  function connect(i){
    if (i === aryServerIP.length) return callback();

    var client = new net.Socket();
    client.connect(rdpport, aryServerIP[i], function(){
      callback(client);
    });
    client.on('error', function(){
      connect(i + 1)
    });
  }
  connect(0)
}

tryConnections(serverip.split(";"), function(client){
  if (client) // Successfully connected to something
  else // all ips failed
});

Another solution would be to use the Async library.

function tryConnections(aryServerIP, callback){
  async.detectSeries(aryServerIP, function(ip, cb){
    var client = new net.Socket();
    client.connect(rdpport, ip, function(){
      cb(client);
    });
    client.on('error', function(){
      cb();
    });
  }, callback);
}