Is JavaScript multithreaded?

Here's my issue - I need to dynamically download several scripts using jQuery.getScript() and execute certain JavaScript code after all the scripts were loaded, so my plan was to do something like this:

function GetScripts(scripts, callback)
{
  var len = scripts.length
  for (var i in scripts)
  {
    jQuery.getScript(scripts[i], function() 
    {
      len --;
      // executing callback function if this is the last script that loaded
      if (len == 0)
        callback()  
    })
  }
}

This will only work reliably if we assume that script.onload events for each script fire and execute sequentially and synchronously, so there would never be a situation when two or more of the event handlers would pass check for (len == 0) and execute callback method.

So my question - is that assumption correct and if not, what's the way to achieve what I am trying to do?

No, JavaScript is not multi-threaded. It is event driven and your assumption of the events firing sequentially (assuming they load sequentially) is what you will see. Your current implementation appears correct. I believe jQuery's .getScript() injects a new <script> tag, which should also force them to load in the correct order.

Currently JavaScript is not multithreaded, but the things will change in near future. There is a new thing in HTML5 called Worker. It allows you to do some job in background.

But it's currently is not supported by all browsers.

The JavaScript (ECMAScript) specification does not define any threading or synchronization mechanisms.

Moreover, the JavaScript engines in our browsers are deliberately single-threaded, in part because allowing more than one UI thread to operate concurrently would open an enormous can of worms. So your assumption and implementation are correct.

As a sidenote, another commenter alluded to the fact that any JavaScriptengine vendor could add threading and synchronization features, or a vendor could enable users to implement those features themselves, as described in this article: Multi-threaded JavaScript?