ObservableProperties.js

var MyClass = function(data) {
	var dataChanging = [],
		dataChanged = [];

	this.data = function(value) {
		if (value !== undefined && value !== data) {
			for (var i = 0; i < dataChanging.length; i++) {
				if (!dataChanging[i](this, value)) {
					return data;
				}
			};
			
			data = value;

			for (var i = 0; i < dataChanging.length; i++) {
				dataChanged[i](this, value);
			};
		}
		return data;
	}

	this.onDataChanging = function (callback) {
		dataChanging.push(callback);
	}

	this.onDataChanged = function(callback) {
		dataChanged.push(callback);
	}
}

var c = new MyClass(5);
c.onDataChanging(function(sender, newValue) {
	if (newValue > 10)
	{
		console.log("'data' should be <= 10");
		return false;
	}
	return true;
});

c.onDataChanged(function(sender, newValue) {
	console.log("'data' changed to " + newValue);
});

c.data(6);
c.data(11);

console.log(c.data());