Figuring Out Media?

I'd like to be able to know, in javascript, if the media that's rendering
is screen or print. I have no nefarious purpose here (I've often seen
people wanting to do this to prevent printing): I have a report that uses
javascript to page 20 items at a time and I want it to show all items at a
time when printing.

- I can make it more or less work on IE using onbeforeprint, but no luck
on better browsers

- When the page first renders the combination of @print and @screen styles
work good, but the moment javascript hides stuff it stays hidden (because
it's changing the element.style attribute of the rows.

- I've tried figuring it out by having an element with different styles
declared in the @print and @screen rules of the stylesheet, but I can't
make it show any difference to the javascript being executed (e.g.
changing the position of this element for print and screen, the element's
position is always what is first calculated for the screen).