View on GitHub


Ring.js - JavaScript Class System with Multiple Inheritance



Use npm to install Ring.js:

npm install ring

Then inside your program:

var ring = require("ring");

Or if you don’t use CommonJS:

<script type="text/javascript" src="node_modules/lodash/lodash.js"></script>
<script type="text/javascript" src="node_modules/ring/ring.js"></script>

Declaring Classes

The ring.create() function is used to declare new classes:

var A = ring.create({
  saySomething: function() {
    return "hello world";
var a = new A();
console.log(a.saySomething()); // prints "hello world" in the JavaScript console

ring.create() takes a dictionary as argument. The content of that dictionary will be added to the prototype of the class.


var A = ring.create({
  constructor: function(name) { = name;
var a = new A("Nicolas");
console.log(; // prints "Nicolas"

When the dictionary of properties given to ring.create() contains a method named constructor, that method will be called during the object construction with the arguments given to the class when using new.


To inherit from one or multiple classes, add a list of the classes to inherit as an argument of ring.create() before the properties dictionary.

var A = ring.create({
  x: function() {
    return "x";
var B = ring.create({
  y: function() {
    return "y";

var C = ring.create([A, B], {});

var c = new C();
console.log(c.x()); // prints "x";
console.log(c.y()); // prints "y";

Diamond inheritance (ie: the classes B and C both inherit from A and the class D inherits from B and C) is allowed by Ring.js.

Alternatively you can use the more classical $extend method:

var A = ring.create({});

var B = A.$extend({});

Super Method

To call the super method when overriding a method, use this.$super(). Pass arguments to this.$super() to forward arguments if necessary.

var A = ring.create({
    sayHello: function(name) {
        return "Hello " + name;
var B = ring.create([A], {
    sayHello: function(name) {
        return this.$super(name) + ", nice to meet you";

var b = new B();
console.log(b.sayHello("Nicolas")); // prints "Hello Nicolas, nice to meet you"

Testing the Class of an Object

The standard procedure in JavaScript to test if an object is an instance of a class is to use instanceof. But, due to the implementation of inheritance in Ring.js (which is necessary to have multiple inheritance), instanceof can not work properly.

So you should use ring.instance() instead.

var A = ring.create({});
var B = ring.create([A], {});

console.log(ring.instance(new A(), A)); // prints "true"
console.log(ring.instance(new B(), A)); // prints "true"

ring.instance(obj, type) will return true if obj is an instance of type or an instance of a sub-class of type.

ring.instance() can also be used with classes and objects that do not use the Ring.js class system. It will use the instanceof operator in that case. It means you can safely replace all the usages of instanceof in any programs by ring.instance() without causing problems.

Additionally, ring.instance() can be used to test the type of basic data types in JavaScript. To do so, use a string as second argument instead of a class. Example:

console.log(ring.instance("", "string")); // prints "true"
console.log(ring.instance(function() {}, "function")); // prints "true"

Creation of New Exception Types

To create new exception types you are supposed to inherit from the standard Error class. But inheriting from that standard class can be hard. That’s why Ring.js contains a feature to bring a solution to that:

var MyError = ring.create([ring.Error], {
    name: "MyError",

try {
    throw new MyError("an error occured");
} catch(e) {
    console.log(ring.instance(e, Error)); // prints "true"
    console.log(ring.instance(e, ring.Error)); // prints "true"
    console.log(ring.instance(e, MyError)); // prints "true"

The ring.Error class is a special class using the Ring.js class system but it also inherits from the standard Error class. You can create new exceptions classes inheriting from it and thus create complex exception hierarchies.

Please note the name property is a property of the standard Error class that can be useful for debugging. So all exception classes should define one.

Compatibility with other class systems

With Ring.js, since version 2, it is possible to inherit from a class created with any other class system. Example with a Backbone model:

var A = Backbone.Model.extend({
    initialize: function() {
        this.a = "a";
var B = ring.create([A], {
    constructor: function() {

Inheriting from a non-Ring class uses the same syntax than with a Ring class. this.$super is usable as always. When overriding the constructor method, simply define a constructor function like you would with a normal Ring class. To call the original implementation of the constructor, use the usual this.$super. It doesn’t matter if you are using a class system that uses another name for the constructor, like initialize, init or whatever. Just use constructor and this.$super() like usual and Ring.js will do the rest.

Also note that, in the above example, new B() instanceof A will return false. instanceof still doesn’t work in this case so you should use ring.instance() instead.

Inheriting from a non-Ring class has some limitations. We could mostly resume it by saying that Ring does not “see” the inheritance in other classes. Example:

var A = ... some non-Ring class ...
var B = ... some non-Ring class inheriting from A ...

var C = ring.create([B], {...});

Instances of C will work as expected in this example. But still, ring doesn’t understand the relation between A and B, and ring.instance(new C(), A) will return false while ring.instance(new C(), B) return true.