[javascript] How do you tell if caps lock is on using JavaScript?

Here is a custom jquery plugin, using jquery ui, made up of all the good ideas on this page and leverages the tooltip widget. The caps lock message is auto applied all password boxes and requires no changes to your current html.

Custom plug in code...

(function ($) {
    $.fn.capsLockAlert = function () {
        return this.each(function () {
            var capsLockOn = false;
            var t = $(this);
            var updateStatus = function () {
                if (capsLockOn) {
                    t.tooltip('open');
                } else {
                    t.tooltip('close');
                }
            }
            t.tooltip({
                items: "input",
                position: { my: "left top", at: "left bottom+10" },
                open: function (event, ui) {
                    ui.tooltip.css({ "min-width": "100px", "white-space": "nowrap" }).addClass('ui-state-error');
                    if (!capsLockOn) t.tooltip('close');
                },
                content: function () {
                    return $('<p style="white-space: nowrap;"/>')
                        .append($('<span class="ui-icon ui-icon-alert" style="display: inline-block; margin-right: 5px; vertical-align: text-top;" />'))
                        .append('Caps Lock On');
                }
            })
            .off("mouseover mouseout")
            .keydown(function (e) {
                if (e.keyCode !== 20) return;
                capsLockOn = !capsLockOn;
                updateStatus();
            })
            .keypress(function (e) {
                var kc = e.which; //get keycode

                var isUp = (kc >= 65 && kc <= 90) ? true : false; // uppercase
                var isLow = (kc >= 97 && kc <= 122) ? true : false; // lowercase
                if (!isUp && !isLow) return; //This isn't a character effected by caps lock

                // event.shiftKey does not seem to be normalized by jQuery(?) for IE8-
                var isShift = (e.shiftKey) ? e.shiftKey : ((kc === 16) ? true : false); // shift is pressed

                // uppercase w/out shift or lowercase with shift == caps lock
                if ((isUp && !isShift) || (isLow && isShift)) {
                    capsLockOn = true;
                } else {
                    capsLockOn = false;
                }
                updateStatus();
            });
        });
    };
})(jQuery);

Apply to all password elements...

$(function () {
    $(":password").capsLockAlert();
});