How to Use the Mouse Wheel Event in HTML5 Pages

Supporting the mouse wheel can add further interactivity to your HTML5 web pages. Rather than scrolling the page, you could perform a different action such as zooming in or out.

View the mouse wheel demonstration page…

Most browsers support the “mousewheel” event for any element. You can register a handling function which is passed an event object. This exposes a wheelDelta property; a positive value for scrolling up or a negative value for scolling down. The larger or smaller the value, the bigger the movement.

Unfortunately, there’s one browser which doesn’t support the the “mousewheel” event. You’re wrong: it’s Firefox. Mozilla has implemented a “DOMMouseScroll” event. This passes an event object with a detail property but, unlike wheelDelta, it returns a positive value for scrolling down. So, until Mozilla adopt the event, we have a bizarre situation where it’s actually a little easier to code for IE6! That’s not something you hear said every day.

You should also note that Apple disable the scroll wheel in Safari, but support is available in the webkit engine so your code won’t cause any problems.

Adding a mousewheel Event Handler

Let’s add an image to our web page which will zoom in and out in response to the mouse wheel:

<img id="myimage" src="myimage.jpg" alt="my image" />

We can now attach the mousewheel event handler:

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

The Cross-Browser mousewheel Event Handling Function

Our MouseWheelHandler can now determine the wheel movement delta. In this case, we’re going to reverse Firefox’s detail value and return either 1 for up or -1 for down:

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)));

We can now size the image accordingly. The code below sets a width between 50px and 800px, but you could determine the image’s natural dimensions and use that.

	myimage.style.width = Math.max(50, Math.min(800, myimage.width + (30 * delta))) + "px";

	return false;
}

Finally, we return false to cancel the standard event which would normally scroll the page.

View the mouse wheel demonstration page…

The code works in every browser, including IE6, 7 and 8, but Safari users won’t see anything happening.

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