Ruby-esque object construction in javascript

rb.js

// kind of like ruby openstruct

var OpenObject = function () {
  this.construct = function (defaults, params) {
    for (var attr in defaults) this[attr] = defaults[attr];
    for (var attr in params) this[attr] = params[attr];
  };
}

var Circle = function (opts) {
  this.construct({
    color: '#ffff00',
    radius: 20
  }, opts);
};
Circle.prototype = new OpenObject();
Circle.prototype.constructor = Circle;

Circle.prototype.toString = function () {
  return 'radius: '+this.radius+' color: '+this.color;
}

var a = new Circle({ color: '#ff00ff' });

console.log(a.toString()); //=> 'radius: 20 color: #ff00ff'

//maybe a slightly more GC sane way?
var Square = function (opts) {
  var DEFAULTS = {
    color: '#ffff00',
    side: 20
  };
  this.construct(DEFAULTS, opts);
}; 
Square.prototype = new OpenObject();
Square.prototype.constructor = Circle;