Delay Loading

How can i get a browser to hold off on loading parts of the html until it has loaded the entire page, so that it doesn't have layers popping up one after another? The problem i've been having is that a button with an onclick js function loads immediately, as its at the top of the html. The js function references a layer further down in the html, and if you click on the button too soon (and it happens all the time while i'm testing as i just click on on the button the moment it loads) js throws an error because that layer does not yet exist. I know i can solve this by having the two layers close to each other in the html, but *something* has to go at the bottom, and there's s much interaction between layers that i'll always have issues with this somewhere...