gistfile1.js

var fs = require('fs');
var async = require('async');

var q = async.queue(function (task, callback){
        timelog('pop');
        fs.appendFile('UIDs.txt', task.value + '\n', function (err) {
                timelog('continue');
                callback();
        });
}, 250);


function millis() {
        return new Date().getTime();
}

function timelog(msg) {
        var a = [].slice.call(arguments);
        msg = '%s: ' + msg;
        a.splice(0, 1, msg, millis());
        console.log.apply(this, a);
}

function wait(ms) {
        var now = millis();
        while (millis() - now < wait) {}
}


/* same function two ways */

function fillSync() {
        timelog('push-sync');
        q.push({'value': i++}, function(err) {});

        if (i == 10) syncComplete = true;
}

function fillAsync() {
        timelog('push-async');
        q.push({'value': i++}, function(err) {});

        if (i == 20) clearInterval(ival);
}

/* synchronous */

var i = 0;

var syncComplete = false;
while(!syncComplete) {
        fillSync();
        wait(1000);
}

/* asynchronous */
var ival = setInterval(fillAsync, 1000);

timelog('complete');