JS to redirect to prev page

I have a page with a stored procedure which is called by different pages.

I want to be able to redirect from that SP page to the page that called the procedure. Whichever is the previous page.

I currently have my SP then at the bottom of the page the following code:

<script language="javascript">
window.location.href="default.asp";
</script>

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but that is harcoded to redirect to that page, and what I need is to redirect to the previous page instead.