/** This is high-level function.
* It must react to delta being more/less than zero.
*/
function handle(delta) {
       if (delta < 0)
…;
       else
…;
}

/** Event handler for mouse wheel event.
*/
function wheel(event){
       var delta = 0;
       if (!event) /* For IE. */
               event = window.event;
       if (event.wheelDelta) { /* IE/Opera. */
               delta = event.wheelDelta/120;
               /** In Opera 9, delta differs in sign as compared to IE.
                */
               if (window.opera)
                       delta = -delta;
       } else if (event.detail) { /** Mozilla case. */
               /** In Mozilla, sign of delta is different than in IE.
                * Also, delta is multiple of 3.
                */
               delta = -event.detail/3;
       }
       /** If delta is nonzero, handle it.
        * Basically, delta is now positive if wheel was scrolled up,
        * and negative, if wheel was scrolled down.
        */
       if (delta)
               handle(delta);
}

/** Initialization code.
* If you use your own event management code, change it as required.
*/
if (window.addEventListener)
       /** DOMMouseScroll is for mozilla. */
       window.addEventListener('DOMMouseScroll', wheel, false);
/** IE/Opera. */
window.onmousewheel = document.onmousewheel = wheel;