An improved observer that tracks changes to existing object properties while still allowing native object access. Should be supported in IE8+, Firefox, Chrome, Safari, etc. (235 bytes minified, gzipped)

observer.v2.js

define(
	'observer',
	[
		'EventEmitter'
	],
	function(EventEmitter) {
		'use strict';

		var OBSERVER_KEY = '_isObserver',
			isStandardDefineProperties = true;

		try {
			Object.defineProperties(new Object(), {});
		} catch(e) {
			// Must be IE8, we'll have to use a DOM Object :\
			isStandardDefineProperties = false;
		}

		function defineProperties(object, options) {
			if (!isStandardDefineProperties) {
				object = document.createElement('div');
			}

			return Object.defineProperties(object, options);
		}

		return function observer(object) {
			var properties = {},
				options = {},
				eventKeys = {},
				key;

			options[OBSERVER_KEY] = {
				value: true,
				writeable: false
			};

			for (key in object) {
				if (typeof object[key] === 'object') {
					object[key] = observer(object[key]);
					eventKeys[key] = object[key];
				}

				properties[key] = object[key];
				options[key] = (function(key) {

					return {
						get: function() {
							return properties[key];
						},
						set: function(value) {
							if (value !== properties[key]) {
								if (typeof value === 'object' && !value[OBSERVER_KEY]) {

									value = observer(value);
								}
								var change = {
									type: 'updated',
									object: this,
									name: key,
									oldValue: properties[key],
									value: value
								};

								properties[key] = value;
								this.trigger('change', change);
							}
						}
					};
				}(key));
			}

			object = EventEmitter(defineProperties(object, options));

			for (key in eventKeys) {
				eventKeys[key].on('change', function(change) {
					change.name = key + '.' + change.name;
					object.trigger('change', change);
				});
			}

			return object;
		};
	}
);