Mousewheel event in modern browsers

Clean and simple:

window.addEventListener("wheel", event => console.info(event.deltaY));

Browsers may return different values for the delta (for instance, Chrome returns +120 (scroll up) or -120 (scroll down). A nice trick to normalize it is to extract its sign, effectively converting it to +1/-1:

window.addEventListener("wheel", event => {
    const delta = Math.sign(event.deltaY);
    console.info(delta);
});

Reference: MDN.

Here's an article that describes this, and gives an example:

http://www.sitepoint.com/html5-javascript-mouse-wheel/

Relevant code, minus the specific example given of resizing an image:

var myitem = document.getElementById("myItem");
if (myitem.addEventListener)
{
    // IE9, Chrome, Safari, Opera
    myitem.addEventListener("mousewheel", MouseWheelHandler, false);
    // Firefox
    myitem.addEventListener("DOMMouseScroll", MouseWheelHandler, false);
}
// IE 6/7/8
else
{
    myitem.attachEvent("onmousewheel", MouseWheelHandler);
}

function MouseWheelHandler(e)
{
    // cross-browser wheel delta
    var e = window.event || e; // old IE support
    var delta = Math.max(-1, Math.min(1, (e.wheelDelta || -e.detail)));

    return false;
}