Custom Checkbox Elements

This idea came about from AOL's use of image checkboxes in the recently launched version of their WebMail product as a means of selecting email messages rather than standard checkboxes.

For those who are not AOL members, here are two screen shots of the interface. Both taken in MSIE, one with images enabled, the other with images disabled. (or "pictures" as they are called in the preferences):

While their solution provides a uniform look across platforms (AOL is very interested in making sure things look "AOLish"), it doesn't do much for accessibility. If images are disabled you've no way to know what the status of the checkbox is -- of course, if you cant even see the images you'll have no way of knowing what their status is either. This is an attempt to address these issues.

How it Works

The images you see above are inserted after each input element who's type attribute is "checkbox" for every form found in the document. The original checkbox has its display set to "none" and all events that are assigned to that checkbox are then assigned to the image. The images are given alt attributes that describe their current status as well.

If the browser has Javascript or images disabled, normal checkboxes are delivered to the user. The latter is accomplished by creating an image element and then calling the so_createCustomCheckBoxes function from its onload event, providing some measure of accessibility.

The checkbox's "checked" status changes with the images, so no additional scripting is required to figure out their status. Submitting the form above will demonstrate this.

This has been tested and verified to work in Firefox 1.0.2, Mozilla 1.7, Safari 1.2.4, MSIE 6, Netscape 7 and Opera 7.5. Suggestions for improvement are welcome.

HTML Source:

<form method="get">
		<label><input type="checkbox" class="chk" name="chkbox1" value="1" />Check Box One</label>
		<label><input type="checkbox" class="chk" name="chkbox2" value="2" />Check Box Two</label>

		<label><input type="checkbox" class="chk" name="chkbox3" value="3" />Check Box Three</label>
		<label><input type="checkbox" class="chk" name="chkbox4" value="4" />Check Box Four</label>
		<label><input type="checkbox" class="chk" name="chkbox5" value="5" />Check Box Five</label>

	<input type="submit" value="test the submit" />

Javascript Source:

window.onload = init;
var d=document;
function init() {

function so_checkCanCreate() {
	// make sure the browser has images turned on. If they are, so_createCustomCheckBoxes will
	// fire when this small test image loads. otherwise, the user will get the hard-coded checkboxes
	testImage = d.body.appendChild(d.createElement("img"));

	// MSIE will cache the test image, causing it to not fire the onload event the next time the
	// page is hit. The parameter on the end will prevent this.
	testImage.src = "blank.gif?" + new Date().valueOf(); = "so_testImage";
	testImage.onload = so_createCustomCheckBoxes;

function so_createCustomCheckBoxes() {
	// bail out is this is an older browser
	// remove our test image from the DOM
	// an array of applicable events that we'll need to carry over to our custom checkbox
	events = new Array("onfocus", "onblur", "onselect", "onchange", "onclick", "ondblclick", "onmousedown", "onmouseup", "onmouseover", "onmousemove", "onmouseout", "onkeypress", "onkeydown", "onkeyup");
	// a reference var to all the forms in the document

	frm = d.getElementsByTagName("form");
	// loop over the length of the forms in the document
	for(i=0;i<frm.length;i++) {
		// reference to the elements of the form
		c = frm[i].elements;
		// loop over the length of those elements
		for(j=0;j<c.length;j++) {
			// if this element is a checkbox, do our thing

			if(c[j].getAttribute("type") == "checkbox") {
				// hide the original checkbox
				c[j].style.position = "absolute";
				c[j].style.left = "-9000px";
				// create the replacement image
				n = d.createElement("img");
				// check if the corresponding checkbox is checked or not. set the
				// status of the image accordingly
				if(c[j].checked == false) {
					n.setAttribute("title","click here to select this option.");
					n.setAttribute("alt","click here to select this option.");
				} else {
					n.setAttribute("title","click here to deselect this option.");
					n.setAttribute("alt","click here to deselect this option.");
				// there are several pieces of data we'll need to know later.
				// assign them as attributes of the image we've created
				// first - the name of the corresponding checkbox
				n.xid = c[j].getAttribute("name");
				// next, the index of the FORM element so we'll know which form object to access later

				n.frmIndex = i;
				// assign the onclick event to the image
				n.onclick = function() { so_toggleCheckBox(this,0);return false; }
				// insert the image into the DOM
				// this attribute is a bit of a hack - we need to know in the event of a label click (for browsers that support it)
				// which image we need turn on or off. So, we set the image as an attribute!
				c[j].objRef = n;
				// assign the checkbox objects event handlers to its replacement image
				for(e=0;e<events.length;e++) if(eval('c[j].' +events[e])) eval('n.' + events[e] + '= c[j].' + events[e]);
				// append our onchange event handler to any existing ones.
				fn = c[j].onchange;
				if(typeof(fn) == "function") {
					c[j].onchange = function() { fn(); so_toggleCheckBox(this.objRef,1); return false; }
				} else {
					c[j].onchange = function () { so_toggleCheckBox(this.objRef,1); return false; }

function so_toggleCheckBox(imgObj,caller) {
	// if caller is 1, this method has been called from the onchange event of the checkbox, which means
	// the user has clicked the label element. Dont change the checked status of the checkbox in this instance
	// or we'll set it to the opposite of what the user wants. caller is 0 if coming from the onclick event of the image
	// reference to the form object
	formObj = d.forms[imgObj.frmIndex];
	// the name of the checkbox we're changing
	objName = imgObj.xid;
	// change the checked status of the checkbox if coming from the onclick of the image
	if(!caller)formObj.elements[objName].checked = !formObj.elements[objName].checked?true:false;
	// finally, update the image to reflect the current state of the checkbox.
	if(imgObj.src.indexOf("chk_on.gif")>-1) {
		imgObj.setAttribute("title","click here to select this option.");
		imgObj.setAttribute("alt","click here to select this option.");
	} else {
		imgObj.setAttribute("title","click here to deselect this option.");
		imgObj.setAttribute("alt","click here to deselect this option.");

slayeroffice Custom Checkbox Demonstration
version 1.0
last revision: 03.30.2005