Menu Won't Resize - It's Static

I have a javascript file that contains methods and the structure to form a popout menu. Then I have another javascript file that uses those methods/structure etc. to build a specific menu with all it's links, titles, etc. To have the menu created on my html page I link to the two .js files and put window.onLoad = writeMenus();

in between script tags that creates the menu right when the user gets to the page. My problem is, since the menu is created when the window loads, it's static. So when I resize my window, the menu won't resize. In my .js files I have the positions of the menu based on screen.width/# so they aren't set values. I think my problem is with using the .onload. It loads the menu once at the beginning but doesn't know to redo it when the window resizes.