Adding cookie-watcher facility in a WebIDL-compliant browser

gistfile1.js

// This code is placed under a CC0 licence

// add a setCookieWatcher global
(function(global){
    "use strict";
    var cookieSetter = Object.getOwnPropertyDescriptor(HTMLDocument.prototype, 'cookie').set;
    var watcher;
    
    Object.defineProperty(HTMLDocument.prototype, 'cookie', {
        set: function(v){
           var oldValue = this.cookie;
           cookieSetter.call(this, v);
           var newValue = this.cookie;

            if(watcher){
                try{
                    watcher(oldValue, newValue);
                }
                catch(e){}
            }

        }
    })

    global.setCookieWatcher = function(w){ watcher = w; }

})(this);

// tests
console.log('cookie', document.cookie);

setCookieWatcher(function(ov, nv){
    console.log('cookie changed from', ov, 'to', nv);
});

document.cookie = 'a';

console.log('cookie', document.cookie);

document.cookie = 'b';

console.log('cookie', document.cookie);