How do I limit the characters in JS

[r0r0]

So I'm using this as a latest post module,

PHP Code:

<script type="text/javascript" src="forums/external.php?type=js"></script> 

<script language="" type="text/javascript">

<!--
for (x = 0; x < 5; x++)
{
document.writeln("<li><a href=\"showthread.php?t="+threads[x].threadid+"\">"+threads[x].title+"</a><span>By: "+threads[x].poster+")</span><br /></li>");
}
//-->
</script> 


What I would like to is limit the thread title to certain amount characters,

so for example:

Welcome to our forums, please read our forum rules before you post.

should output,

Welcome to our forums, please read our forum...

Thanks