This past week, Jeremy Ashkenas (of CoffeeScript fame) started a flurry of discussion around class syntax for JavaScript. ECMAScript Harmony is scheduled to have classes and the proposal has been up for a while. Of course, JavaScript has never had a true concept of classes (which is why I call them “types” instead), and the current strawman is no exception – it simply creates some syntactic sugar on top of the current constructor/prototype method of defining custom types. An example:

class Color {

  constructor(hex) {
    ...
  }

  public r = 1;
  public g = 1;
  public b = 1;

  copy(color) {
    ...
  }

  setRGB(r, g, b) {
    ...
  }

  setHSV(h, s, v) {
    ...
  }

}

This would be instead of defining a separate constructor and prototype. The above desugars to:

function Color(hex){
    ...
}


Color.prototype.r = 1;
Color.prototype.g = 1;
Color.prototype.b = 1;

Color.prototype.copy = function(color){
    ...
};

Color.prototype.setRGB = function(r,g,b){
    ...
};

Color.prototype.setHSV = function(h,s,v){
    ...
};

Essentially the new class syntax just helps you define the prototype of the new type while the constructor is responsible for creating instance members.

Jeremy didn’t like it, and so came up with an alternate proposal in the form of a gist. At the center of his idea: use the familiar object literal syntax to define new types with just a small amount of syntactic sugar to make things easier.

class Color {

  constructor: function(hex) {
    ...
  },

  r: 1, g: 1, b: 1,

  copy: function(color) {
    ...
  },

  setRGB: function(r, g, b) {
    ...
  },

  setHSV: function(h, s, v) {
    ...
  }

}

Jeremy’s proposal looks closer to object literal syntax with the class keyword and the type name. A lot of commenters on the gist liked this idea – I’m actually not one of them, I think the proposed Harmony syntax is much more succinct and implements sugaring of known patterns in a straightforward way.

Regardless, there is something to Jeremy’s approach of being able to define new custom types in one step. It’s pretty trivial to do that today using JavaScript. First, you need a simple function:

function type(details){
    details.constructor.prototype = details;
    return details.constructor;
}

That’s all it takes. Basic usage:

var Color = type({
     constructor: function(hex) {
         ...
     },

     r: 1, g: 1, b: 1,

     copy: function(color) {
         ...
     },

     setRGB: function(r, g, b) {
         ...
     },

     setHSV: function(h, s, v) {
         ...
     }
});

var mycolor = new Color("ffffff");

The syntax is just a bit different from Jeremy’s as it adheres to ECMAScript 5 syntax, but works pretty much the same way. The key to understanding this approach is understanding the constructor property. You may be used to accessing constructor from an object instance to get the function that created the object. However, constructor is actually a prototype property, shared by all instances. For any given function created from scratch:

function f(){}
console.log(f === f.prototype.constructor);   //true

So basically, the type() function takes the passed-in object and looks for the constructor property. At first, details.constructor.prototype has its default value. The function overwrites the prototype with the details object itself (which already has an appropriate reference to constructor). Then, it simply returns the now-fully-formed constructor function. You can start to use the returned constructor with new immediately.

In lieu of Harmony’s new syntax, I’ve very quickly come to like this approach. Using a single object literal is quick and easy, and of course, works right now in all browsers. There are also any number of ways you could modify type() in order to support things like inheritance and mixins, depending on your use cases.

In the end, I’m looking forward to having some syntactic sugar for defining custom types in JavaScript. We’ve battled for too long with overly-verbose composition statements while those using class-based languages looked over our shoulders and laughed. I, for one, welcome our new Harmony overlords.

Update (04-Nov-2011): Fixed Harmony example.