I’ve been doing a lot of work, lately, with JavaScript inheritance – namely for my work-in-progress JavaScript book – and in doing so have examined a number of different JavaScript classical-inheritance-simulating techniques. Out of all the ones that I’ve looked at I think my favorites were the implementations employed by base2 and Prototype.

I wanted to go about extracting the soul of these techniques into a simple, re-usable, form that could be easily understood and didn’t have any dependencies. Additionally I wanted the result to be simple and highly usable. Here’s an example of what you can do with it:

var Person = Class.extend({ init: function(isDancing){ this.dancing = isDancing; }, dance: function(){ return this.dancing; } }); var Ninja = Person.extend({ init: function(){ this._super( false ); }, dance: function(){ // Call the inherited version of dance() return this._super(); }, swingSword: function(){ return true; } }); var p = new Person(true); p.dance(); // => true var n = new Ninja(); n.dance(); // => false n.swingSword(); // => true // Should all be true p instanceof Person && p instanceof Class && n instanceof Ninja && n instanceof Person && n instanceof Class

A couple things to note about this implementation:

Creating a constructor had to be simple (in this case simply providing an init method does the trick).

In order to create a new ‘class’ you must extend (sub-class) an existing class.

All of the ‘classes’ inherit from a single ancestor: Class. Therefore if you want to create a brand new class it must be a sub-class of Class.

And the most challenging one: Access to overridden methods had to be provided (with their context properly set). You can see this with the use of this._super() , above, calling the original init() and dance() methods of the Person super-class.

I’m pleased with the result: It helps to enforce the notion of ‘classes’ as a structure, maintains simple inheritance, and allows for the super method calling.

Simple Class Creation and Inheritance

And here’s the implementation (reasonably sized and commented well) – clocking in at around 25 lines. Feedback is welcome and appreciated.

/* Simple JavaScript Inheritance * By John Resig https://johnresig.com/ * MIT Licensed. */ // Inspired by base2 and Prototype (function(){ var initializing = false, fnTest = /xyz/.test(function(){xyz;}) ? /\b_super\b/ : /.*/; // The base Class implementation (does nothing) this.Class = function(){}; // Create a new Class that inherits from this class Class.extend = function(prop) { var _super = this.prototype; // Instantiate a base class (but only create the instance, // don't run the init constructor) initializing = true; var prototype = new this(); initializing = false; // Copy the properties over onto the new prototype for (var name in prop) { // Check if we're overwriting an existing function prototype[name] = typeof prop[name] == "function" && typeof _super[name] == "function" && fnTest.test(prop[name]) ? (function(name, fn){ return function() { var tmp = this._super; // Add a new ._super() method that is the same method // but on the super-class this._super = _super[name]; // The method only need to be bound temporarily, so we // remove it when we're done executing var ret = fn.apply(this, arguments); this._super = tmp; return ret; }; })(name, prop[name]) : prop[name]; } // The dummy class constructor function Class() { // All construction is actually done in the init method if ( !initializing && this.init ) this.init.apply(this, arguments); } // Populate our constructed prototype object Class.prototype = prototype; // Enforce the constructor to be what we expect Class.prototype.constructor = Class; // And make this class extendable Class.extend = arguments.callee; return Class; }; })();

In my opinion the two trickiest parts are the “initializing/don’t call init” and “create _super method” portions. I want to cover those briefly so that you will have a good understanding of what’s being achieved in this method.

Initialization

In order to simulate inheritance with a function prototype we use the traditional technique of creating an instance of the super-class function and assigning it to the prototype. Without using the above it would look something like this:

function Person(){} function Ninja(){} Ninja.prototype = new Person(); // Allows for instanceof to work: (new Ninja()) instanceof Person

What’s challenging about this, though, is that all we really want is the benefits of ‘instanceof’, not the whole cost of instantiating a Person object and running its constructor. To counteract this we have a variable in our code, initializing , that is set to true whenever we want to instantiate a class with the sole purpose of using it for a prototype.

Thus when it comes time to actually construct the function we make sure that we’re not in an initialization mode and run the init method accordingly:

if ( !initializing ) this.init.apply(this, arguments);

What’s especially important about this is that the init method could be running all sorts of costly startup code (connecting to a server, creating DOM elements, who knows) so circumventing this ends up working quite well.

Super Method

When you’re doing inheritance, creating a class that inherits functionality from a super-class, a frequent desire is the ability to access a method that you’ve overridden. The final result, in this particular implementation, is a new temporary method ( ._super ) which is only accessible from within a sub-classes’ method, referencing the super-classes’ associated method.

For example, if you wanted to call a super-classes’ constructor you could do that with this technique.

var Person = Class.extend({ init: function(isDancing){ this.dancing = isDancing; } }); var Ninja = Person.extend({ init: function(){ this._super( false ); } }); var p = new Person(true); p.dancing; // => true var n = new Ninja(); n.dancing; // => false

Implementing this functionality is a multi-step process. To start, note the object literal that we’re using to extend an existing class (such as the one being passed in to Person.extend ) needs to be merged on to the base new Person instance (the construction of which was described previously). During this merge we do a simple check: Is the property that we’re attempting merge a function and is what we’re replacing also a function? If that’s the case then we need to go about creating a way for our super method to work.

Note that we create an anonymous closure (which returns a function) that will encapsulate the new super-enhanced method. To start we need to be a good citizen and save a reference to the old this._super (disregarding if it actually exists) and restore it after we’re done. This will help for the case where a variable with the same name already exists (don’t want to accidentally blow it away).

Next we create the new _super method, which is just a reference to the method that exists on the super-class’ prototype. Thankfully we don’t have to make any additional changes, or re-scoping, here as the context of the function will be set automatically when it’s a property of our object ( this will refer to our instance as opposed to the super-class’).

Finally we call our original method, it does its work (possibly making use of _super as well) after which we restore _super to its original state and return from the function.

Now there’s a number of ways in which a similar result, to the above, could be achieved (I’ve seen implementations that have bound the super method to the method itself, accessible from arguments.callee ) but I feel that this technique provides the best mix of usability and simplicity.

I’ll be covering a lot more of the nitty-gritty behind the JavaScript prototype system in my completed work but I just wanted to get this Class implementation out there to get everyone trying it out and playing with it. I think there’s a lot to be said for simplistic code (easier to learn, easier to extend, less to download) so I think this implementation is a good place to start and learn the fundamentals of JavaScript class construction and inheritance.

This topic will be discussed, in depth, in my work-in-progress book: Secrets of the JavaScript Ninja. To be released Fall 2008.