How do I get javascript to work in BBcode?

[skepticality]

I trying to implement a BB-code that uses java-script, but I can't get it to work.

I wanted to make an [IMG]-tag that restricted the size of the image to a pre-defined width.
Both code-examples works like a charm when I have them in ordinary HTML-files on my computer.

When I insert the first code, weird things happens. When I load the page, the tag does not resize the image to the restricted size, but it displays the image. However, if I enter edit-mode, and then save the post, the images get resized. In my code example below, width is restricted to 500px.

<script type="text/javascript">
function largerThan(a,b)
{
return a>b;
}
</script> 
 
<img src="{param}" onload=if(largerThan(this.width,500)){{this.height=this.height*500/this.width}{this.width=500}};>

The second code that is supposed to manage this diplays nothing at all. No BBcode tags are visible in the post, no photos either.

<script language="javascript">
var url = '{param}';
var maxwidth = 500;
 
var newImg = new Image();
newImg.src = url;
var width = newImg.width;
if (width > maxwidth){ width=maxwidth; }
 
document.write('<a href='+url+'><img src='+url+' width='+width+' border=0></a>');
 
</script>

regards,
Dr. Mabuse
Admin, Skepticality Forum