javascript - finding the maximum scroll position of a page

Question

I have made the body of the page 200% tall so that it fits on a screen twice. Using javascript I am making it keep scrolling to the top or bottom when you scroll. For this, I need to find out the lowest scroll point of the page on any browser or screen size so that it stops when it gets there.

No JQuery please.
Thank you.

My code: (it is still being put together so needs a bit of work)

function getScrollXY() {
    var x = 0, y = 0;
    if( typeof( window.pageYOffset ) == 'number' ) {
        // Netscape
        x = window.pageXOffset;
        y = window.pageYOffset;
    } else if( document.body && ( document.body.scrollLeft || document.body.scrollTop ) ) {
        // DOM
        x = document.body.scrollLeft;
        y = document.body.scrollTop;
    } else if( document.documentElement && ( document.documentElement.scrollLeft || document.documentElement.scrollTop ) ) {
        // IE6 standards compliant mode
        x = document.documentElement.scrollLeft;
        y = document.documentElement.scrollTop;
    }
    return [x, y];
}

function scrollup() {
    if (xy[1] > 0) {
        window.scrollBy(0,-100);
        setTimeout(scrollup,200);
    } else {
        null;
    }
}

function scrolldown() {
    if (xy[1] < ) {
        window.scrollBy(0,100);
        setTimeout(scrolldown,200);
    } else {
        null;
    }
}

function dothescroll() {
    var xy = getScrollXY();
    var y = xy[1];
    setTimeout(function(){
        if (xy[1] > y) {
            scrollup();
        } else {
            scrolldown();
        }
    },200);
}

Answer 1

This is the cross browser compatible version:

var limit = Math.max( document.body.scrollHeight, document.body.offsetHeight, 
                   document.documentElement.clientHeight, document.documentElement.scrollHeight, document.documentElement.offsetHeight );