You can include a javascript like this:

function include(filename)
 {

var head = document.getElementsByTagName('head')[0];
 script = document.createElement('script');
 script.src = filename; 
script.type = 'text/javascript';
 head.appendChild(script)

}
Read more here: http://forums.digitalpoint.com/showthread.php?t=146094

Or even better:
You can easily read another JS-file inside the javascript using jquery:

$.getScript("my_lovely_script.js", function(){
    alert("Script loaded and executed.");    
  // here you can use anything you defined in the loaded script 
});

Read here: http://stackoverflow.com/questions/950087/include-javascript-file-inside-javascript-file

A common problem when programming newsletter software is how to track how many – or even who – opens / read the newsletter. Specifically how many reads it is very hard to know, but it IS possible to see how many opens your newsletter/mailinglist message – at least if you use images in your newsletter.

The trick is to track downloads of a certain image.

When ever this image is opened, the “counter” just adds 1 to the number of times the newsletter have been read/opened. Of course this requieres e.g. Outlook-users to accept that pictures should be shown, but then again if they don’t they probably don’t actually read the message and might as well not be included in the count.

If you setup a system that automates this so each reciever of the newsletter has his own “image”-code, you can even track who specifically read (opens that is) you newsletter.

The reciepe could be something like:
<img src=”http://mydomain.com/track.php?ID=newsletter-december-2009″ width=”0″ height=”0″ />
OR
<img alt=logo” src=”http://mydomain.com/track.php?ID=000001″ width=”500″ height=”100″ />
.. or a combination of the two.

If using the second model, where the image is actually visible, you need to make sure that the webserver sends an actual image when track.php is “asked”.

(Thanks to http://www.webmasterworld.com/forum10/5538.htm for leading me on to this to begin with.)