after.js

function ImmutableRectangle(){
  this.getWidth = function(w){
    console.debug('上書きできるぜ！');
  }
}
ImmutableRectangle.prototype.getWidth = function(w){
  console.debug('prototypeの定義だぜ', w);
}

var r = new ImmutableRectangle();
r.getWidth(100);