Why use document.write?

Question

I was wondering why ad's still use the document.write approach to inserting the add into the page

<script language="javascript" type="text/javascript">
    document.write("<script type='text/javascript' src='http://addomain/someadd.js'><\/sc" + "ript>");
</script>

Why is it that I can't just put

<script type='text/javascript' src='http://addomain/someadd.js'></script>

In place of the ad?

Answer 1

A traditional script tag will block the page while it is loading and executing. A script loaded with document.write will work asynchronously. That's why you see this on ads or analytics, as such scripts don't influence the page content directly.

Answer 2

I work with a web advertising company, and from what I've heard, certain browsers (don't know off hand which ones) will allow you to drop script tags into the page, but won't allow you to automatically execute their contents.

So, to pull this off, you need to break the script tag into pieces so the browser doesn't treat it as a script tag, but rather as any old HTML Data. Then, as the DOM is processed serially, the next thing it evaluates, after writing out the script tag is... hey, that script tag you just wrote out.

At this point the script tag is evaluated and executed.