gistfile1.js

function MyConstructor() {
	var instance = function() {
		alert('yey');
	}
	
	instance.constructor = MyConstructor;
	instance.__proto__ = MyConstructor.prototype;
	
	return instance;
}

MyConstructor.prototype.instanceMethod = function() {
	alert('woo');
}

var myInstance = new MyConstructor;

alert(myInstance instanceof MyConstructor); // true
myInstance.instanceMethod(); // woo
myInstance(); // yey