Reading JavaScript Patterns I thought of another way to write the decorator pattern. Not yet sure if this has any pitfalls.

gistfile1.js

SalesOrder.prototype.decorate = function(decorator) {
  var override
  for (override in decorator) {
    if (decorator.hasOwnProperty(override)) {

      var oldfunc = this[override]
      var decfunc = decorator[override]

      var newfunc = function() {
        var args = Array.prototype.slice.call(arguments)
        return decfunc.call(this, oldfunc.apply(this, args))                       
      }   

      this[override] = newfunc
    }   
  }

}

SalesOrder.decorators = {}
SalesOrder.decorators.tax = { 
  getPrice: function(p) {
    return p * 1.08
  }
}

function SalesOrder() {
  this.getPrice = function() {
    return 12.88
  }
}

//var sale = new SalesOrder()
//sale.decorate(SalesOrder.decorators.tax)
//console.log(sale.getPrice())