Calling a javascript function disables calendar extender

I have a script manager and calendar extender on my aspx page. When I call a javascript function from my code behind page, the calendar extender doesn't work
anymore. I call it like this:
ScriptManager.RegisterStartupScript(Page, this.GetType(), "javascriptfunction", "<script language=javascript>function();</script>", true);
OR
Page.ClientScript.RegisterStartupScript(this.GetType(), "javascriptfunction", "<script language=javascript>function();</script>", true);

Any ideas?

Thank you.

There may be an error in your JS. Check JS error.
Which version u r using?

Thanks for your reply. The javascript doesn't seem to error as it accomplishes what it's supposed to. The javascript function in the .aspx page starts as
window.onload = function () {
code here....
}

and them I'm calling it in the gridview databound event:
ScriptManager.RegisterStartupScript(Page, this.GetType(), "javascriptfunction", "<script language=javascript>function();</script>", true);

You can try using these on page_loadcomplete
CalendarExtender.EnsureValid
CalendarExtender.Databind