gistfile1.js

var DatabaseFactory = {
	getInstanceById: function(id, type) {
		if(!this.instances) this.instances = {};
		
		var theId = id + "" + typeof type;
		if(this.instances[theId]) {
			return this.instances[theId];
		} else {
			this.instances[theId] = new type(id);
			return this.instances[theId]; 
		}
	}
}

// Database object
function Database(id) {
	this.id = id;
}

Database.prototype.save = function() {
	console.log("save");
}

Database.prototype.delete = function() {
	console.log("delete " + this.id);
}

// User Object
function User(id) { this.constructor(id) };

User.prototype = new Database();
User.prototype.parent = Database.prototype;

User.prototype.test = function() {
	console.log("test");
}

User.prototype.delete = function() {
	console.log("delete user");
	this.parent.delete.call(this);
}

// Test
var myObj = DatabaseFactory.getInstanceById(2, User);
myObj.save();
myObj.test();
console.log(myObj.id);
myObj.delete();