A cross-browser JS function to get the viewport's scroll top value (http://stackoverflow.com/questions/871399/cross-browser-method-for-detecting-the-scrolltop-of-the-browser-window)

scrolltop.js

function getScrollTop(){
    if(typeof pageYOffset!= 'undefined'){
        //most browsers except IE before #9
        return pageYOffset;
    }
    else{
        var B= document.body; //IE 'quirks'
        var D= document.documentElement; //IE with doctype
        D= (D.clientHeight)? D: B;
        return D.scrollTop;
    }
}

alert(getScrollTop());