Function that watches the property of any object for changes.

watch.js

/**
 * Standalone function used to continuously watch the property of an object for changes.
 *
 * The function is as optimized as possible to reduce needless timeout and interval callbacks, but this should still be
 * used sparingly and only when events are not possible.
 *
 * Usage:
 *      var item = watch(someObject, "propertyName", function(oldValue, newValue, cancel) {
 *          // Handle the change here.
 *          // Optionally cancel watching.
 *          cancel();
 *      });
 *
 *      // You can also cancel with the return value.
 *      item.cancel();
 */
window.watch = (function() {
    var watchList = [], timer = null, timerResolution = 500;

    function WatchItem(object, property, callback) {
        this.object = object;
        this.property = property;
        this.callback = callback;
        this.active = true;
        this.oldValue = object[property];
    }

    WatchItem.prototype.cancel = function() {
        this.active = false;

        var index = watchList.indexOf(this);
        if (index >= 0) {
            watchList.splice(index, 1);
        }

        if (watchList.length === 0) {
            clearInterval(timer);
            timer = null;
        }
    };

    function sentinel() {
        for (var i = 0; i < watchList.length; ++i) {
            var item = watchList[i];
            var newValue = item.object[item.property];

            if (newValue !== item.oldValue) {
                item.callback(item.oldValue, newValue, item.cancel);
            }

            if (!item.active) {
                --i;
            }

            item.oldValue = newValue;
        }
    }

    return function(object, property, callback) {
        var item = new WatchItem(object, property, callback);
        watchList.push(item);

        if (timer === null) {
            timer = setInterval(sentinel, timerResolution);
        }

        return item;
    };
})();

Don't use this, it's bad for performance and not very responsive. ;)