Can the same js function make AJAX calls at the same time?

The problem is that you're using a single global variable to store all your XMLHttpRequest instances. Whenever you create a new instance, and store its reference in the global variable, xmlhttp, the instance that was previously referenced there becomes un-referenced, and so it will be garbage collected as soon as the browser sees fit. Usually, this means it is garbage collected immediately; and in your case, it is being garbage collected even before the pending response is received.

One way to fix this problem, is to declare xmlhttp as a local var within your update_revision() function. Like this:

function update_revision(revision, server_name) { // declare xmlhttp locally var xmlhttp; if (window.XMLHttpRequest) { // code for IE7+, Firefox, Chrome, Opera, Safari xmlhttp = new XMLHttpRequest(); } else { // code for IE6, IE5 xmlhttp = new ActiveXObject("Microsoft.XMLHTTP"); } xmlhttp.onreadystatechange = function() { if (xmlhttp.readyState == 4 ) { if( xmlhttp.status == 200) { document.getElementById("rev." + revision).value = "updated to " + update_to; } // explicitly drop the reference when // we're done with it, so the browser will reclaim the object // (this is optional, but it's a good idea) xmlhttp = null; } } xmlhttp.open("GET", "https://" + server_name + "/imacs/radek/svn_update.php?code_base=" + revision + "&revision=" + update_to + "&t=" + Math.random(), true); xmlhttp.send(); } 

NOTES:

1. xmlhttp is declared as a local var

2. xmlhttp is "nulled" when we don't need it anymore (after readyState == 4). This will cause the browser to garbage collect the request instance, thus preventing resource leaking. This is a good practice (and so I've shown it in the example above), but technically, it's optional.