Constructor function as class in JavaScript.

constructor_function_as_class.js

// type declaration
var A = makeType({
	init: function(x, y){
		this.x = x;
		this.y = y;
	},

	hello: function(){
		console.log(this.x, this.y)
	}
});

// B is subclass of A
var B = makeType(A, {
	init: function(x, y, z){
		// call super
		Object.getPrototypeOf(B.prototype).init.call(this, x, y);
		this.z = z;
	},
	run: function(){
		console.log("running", this.z)
	}
});


function makeType(){
	var base, declaration;
	if(typeof arguments[0] == "function"){
		base = arguments[0];
		declaration = arguments[1];
	} else {
		declaration = arguments[0];
	}

	function Ctor(){
		var self = this;
		if(!(this instanceof Ctor)){
			self = Object.create(Ctor.prototype);
		}
		self.init.apply(self, arguments);
		return self;
	}

	var proto = Object.create((base && base.prototype) || Object.prototype);
	Object.getOwnPropertyNames(declaration).forEach(function(propName){
		proto[propName] = declaration[propName];
	});
	Ctor.prototype = proto;

	return Ctor;
}

// usage
var assert = require("assert");

// create using new
var a1 = new A(1, 2);
assert.ok(a1 instanceof A);
assert.ok(A.prototype.isPrototypeOf(a1));
assert.notEqual(A.prototype.constructor, A);
a1.hello();

// new does not matter
var a2 = A(1, 2);
assert.ok(a2 instanceof A);
assert.ok(A.prototype.isPrototypeOf(a2));
assert.notEqual(A.prototype.constructor, A);
a2.hello();

// check B which is a subclass of A
var b1 = new B(1, 2, 3);
assert.ok(b1 instanceof B);
assert.ok(b1 instanceof A);
b1.hello();
b1.run();