Cross-browser method for detecting the scrollTop of the browser window

If you don't want to include a whole JavaScript library, you can often extract the bits you want from one.

For example, this is essentially how jQuery implements a cross-browser scroll(Top|Left):

function getScroll(method, element) {
  // The passed in `method` value should be 'Top' or 'Left'
  method = 'scroll' + method;
  return (element == window || element == document) ? (
    self[(method == 'scrollTop') ? 'pageYOffset' : 'pageXOffset'] ||
    (browserSupportsBoxModel && document.documentElement[method]) ||
    document.body[method]
  ) : element[method];
}
getScroll('Top', element);
getScroll('Left', element);

Note: you'll notice that the above code contains a browserSupportsBoxModel variable which is undefined. jQuery defines this by temporarily adding a div to the page and then measuring some attributes in order to determine whether the browser correctly implements the box model. As you can imagine this flag checks for IE. Specifically, it checks for IE 6 or 7 in quirks mode. Since the detection is rather complex, I've left it as an exercise for you ;-), assuming you have already used browser feature detection elsewhere in your code.

Edit: If you haven't guessed already, I strongly suggest you use a library for these sorts of things. The overhead is a small price to pay for robust and future-proof code and anyone would be much more productive with a cross-browser framework to build upon. (As opposed to spending countless hours banging your head against IE).

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())

Or just simple as:

var scrollTop = document.body.scrollTop || document.documentElement.scrollTop;