How Can I Ensure This Loop Runs As Expected In Node.js

The following code outputs the port number 40001 twice, then exits with error: Error: listen EADDRINUSE I believe this is because the code is asynchronous and does not wait for on

Solution 1:

So you are right. The problem here is that when you find the port on the first iteration (40001) you don't consume it before the next iteration, so 40001 is found twice.

So there are 2 approaches here. Either you use a continuation, i.e. you create a next step to perform as a function, and then call the continuation after you have consumed the port.

In this case it's probably easier just to keep a track of the port though. Start by defining a base port at 40000, when you find a port, set the base port to freeport + 1 and search on that range.

var startPort = 40000var endPort = 60000

posts.forEach(function(element, index, posts){

    portscanner.findAPortNotInUse(startPort, endPort, 'localhost', function(err, freeport) {
        if(err) {
            returnconsole.log(err);
        }
        console.log(freeport);

        startPort = freeport + 1// correct the start port to search from

        phantom.create({'port': freeport}, function(ph){

            return ph.createPage(function(page) {
                return page.open("http://www.target.com/showthread.php?t="+posts[index].post_id, function(status) {
                    console.log("opened post? ", status);

                    return page.get('plainText', function(content){
                        console.log(content);
                        return ph.exit();
                    });
                });
            });
        });
    });
});

Continuation using continuable-series (not tested but should give you an idea)

var phantom = require("phantom")
var portscanner = require("portscanner")
var series = require("continuable-series");

module.exports = functionprocessPosts(posts) {

    var callInSeries = posts.map(function (post) {
        return phantomPage.bind(null, post)
    });

    series(callInSeries, function (err) {
        if (err) {
            console.log(err)
        }
    }
}    

functionphantomPage(post, callback) {
    portscanner.findAPortNotInUse(40000, 60000, 'localhost', function(err, freeport) {
        if (err) {
            returncallback(err);
        }

        phantom.create({'port': freeport}, function(ph){
            // Now that we have opened the port, we can callback immediately// starting the next iterationcallback(null)

            return ph.createPage(function(page) {
                return page.open("http://www.target.com/showthread.php?t="+post.post_id, function(status) {
                    console.log("opened post? ", status);

                    return page.get('plainText', function(content){
                        console.log(content);
                        return ph.exit();
                    });
                });
            });
        });
    });
}

Solution 2:

I recently ran into the same issue and fixed it by using Math.Random to generate a start port value and just added 500 to get the end port value.

functiongetRandomInt(min, max) {
    min = Math.ceil(min);
    max = Math.floor(max);
    return Math.floor(Math.random() * (max - min)) + min;
}

var startPort = getRandomInt(3000, 8000);
var endPort = startPort + 500;

portscan.findAPortNotInUse(startPort, endPort, '127.0.0.1', function(err, ports) {
   .....
}