test_new.js

function Rect(width, height) {
    this.width = width;
    this.height = height;
}
Rect.prototype.type = function() { return "rect"; };

function Square(width) {
    return new Rect(width, width);
}
Square.prototype.type = function() { return "square"; };

sq = new Square(12);

log(sq.type()); //=> ???
log(sq instanceof Square); //=> ???