Tonight I was experimenting a bit with a minimalist implementation of advising in Aspect Oriented Javascript. I Implemented the three basic advices (before, around and after). For this naive implementation I slammed the functions in the Object's prototype. This way I had access to them in all objects in the system.

Here's how to use them:

Dog.before('bark',function(){alert('going to bark')});
Dog.after('bark',function(){alert('done barking')});
User.prototype.before('login',function(){alert('logging in!')}

The actual code written is very small (20 lines, less if you discount lines taken by braces)

Object.prototype.before = function(func, advice){
var oldfunc = this[func];
this[func] = function(){
   advice.apply(this,arguments);
   oldfunc.apply(this,arguments);
}
}

Object.prototype.around = function(func, advice){
   var oldfunc = this[func];
   this[func] = function(){
      var myargs = [oldfunc];
      for(var i=0; i < arguments.length;i++){
         myargs.push(arguments[i])
      }
      advice.apply(this,myargs);
   }
}

Object.prototype.after = function(func, advice){
var oldfunc = this[func];
this[func] = function(){
   oldfunc.apply(this,arguments);
   advice.apply(this,arguments);
}
}

This way you can add any sorts of filters to your JavaScript methods. Enhancing on this is to be able to remove those filters once added and to add a filter to all functions of an object (recursively) at once.

We also need some fail safety against users trying to advice non functions or even undefined properties.