Introduction

What is it ?

This is an example on how to detect which element scrolls the view (HTML or BODY). It started with a feedback request on a JSMentors question/thread about smooth scrolling.

The 'game' was transitioning the scrollTop property using requestAnimationFrame (rAF). At that point I was tempted and curious to see how many lines of code were realy necessary for that task.

TODO:

• make the duration time configurable (currently fixed to 1000ms)
• allow referencing external easing functions to improve smoothness
• improve code and readibility, add code comments were necessary
• Bugs ? Please report !

Is it of some use ?

It depends on your needs, from this small code you can just take the getScrollingElement() if it work for you.

However, you may also look at the code and see how to profit a global capturing event (delegation). Also see how adding a few lines of code would solve one of the most frequent web problems: perform operations when users click on selected page links.

The advantages of the final approach are:

• avoid using monster frameworks
• avoid waiting for a ready/load event
• avoid attaching listener to hundreds elements
• 60fps smooth scroll using native requestAnimationFrame
• options let the target anchor be set/followed in the URL
• pure Javascript, no libraries, full cross-browser down to IE6
• faster page load and faster running scripts, no memory wasted

How can I use/profit it ?

The getScrollingElement() function performs a feature detection to find which element is responsible for scrollling the current view.

Firefox, Opera and IE use the document.documentElement when in Standard mode and the document.body when in Quirks mode.

Webkit, on his side and disrespectful of current specifications, will always use the document.body (W3C specifications CSSOM View Module).

The feature detection, if proven to work, could avoid DOM code to fail when Webkit will fix the known and long-standing scrolling bug.

Currently DOM libraries and frameworks use a regular expression to sniff the UA and exclude Webkit from using the document.documentElement but that assumption may fail soon. A correct feature detection test is needed, hope this fits for the task.

Cut & Paste the following feature detection function in your code:

    function getScrollingElement() {
        var d = document;
        return  d.documentElement.scrollHeight > d.body.scrollHeight &&
                d.compatMode.indexOf('CSS1') == 0 ?
                d.documentElement :
                d.body;
    }

and invoke it without arguments:

    var scrollEl = getScrollingElement();

Needed info

getScrollingElement() must be invoked every time, once, just before starting a scroll (content may change).

If you do tests with it make sure there is enough content to be scrolled (vertical scrollbar should be visible) or nothing will happen.