change src of iframe before it loads

[ofir]

I have a page with iframe that i can only reach to it's attached .js
file.
i'm trying to change the src of the iframe but the only this i found
is:
document.attachEvent("onreadystatechange",startup)
function startup()
{
try{
var iframes = document.getElementsByTagName("IFRAME");
iframes(0).src = "http://www.blablabla.com";
catch(e){}
}

it worked but only after it allready loads the content of the iframe
so it first loads the original src and then my new one.
can I attach to an event that run before the original src loads into
the iframe?

thanks,

 

[ofir]

the .js is attached to the parent document.
and about what you said "something smells rotten", it isn't. this is
an application that gave me control only on that .js but the page it
self and the iframe page are from code behind. that's it

 

[ofir]

the js file is in the parent document. and about what you said
"something smells rotten" you wrong. this is an application that
allows me to change the js file only but the other content comes from
code behind(the parent html and the iframe html)