Proxying of document.cookie

Question

I need to log setting of document.cookie. I can not redefine cookie property just with document.cookie = {...} So I need to get setter for document.cookie. But Object.getOwnPropertyDescriptor(document, "cookie") returns undefined.

UPD. While I was writing the question I found a working solution, but it uses deprecated __lookupGetter__ and __lookupSetter__ methods. Is there any solution which doesn't use obsolete API?

Answer 1

The standardized way of accessing getters and setters is with Object.getOwnPropertyDescriptor, but as the name suggests, it only looks on the objects own properties (it does not look up the prototype chain). document is an instance of HTMLDocument, which inherits from Document. In modern browsers the cookie property is defined on Document.prototype, whereas in older versions of Firefox it is defined on HTMLDocument.prototype.

var cookieDesc = Object.getOwnPropertyDescriptor(Document.prototype, 'cookie') ||
                 Object.getOwnPropertyDescriptor(HTMLDocument.prototype, 'cookie');
if (cookieDesc && cookieDesc.configurable) {
    Object.defineProperty(document, 'cookie', {
        get: function () {
            return cookieDesc.get.call(document);
        },
        set: function (val) {
            console.log(val);
            cookieDesc.set.call(document, val);
        }
    });
}

Ironically, in the most privacy-concerned browser Safari, the descriptor has set configurable to false and does not contain the getter nor setter, and neither does __lookupGetter__ or __lookupSetter__. So I haven't found a way to override document.cookie in Safari yet (8.0.8 on OS X and iOS 9.0.2). WebKit nightly acts the same way as Safari, so it doesn't seem to get fixed anytime soon.

Update October 2019: Tested the above code in Safari 12.1.2 on MacOS Mojave, and cookieDesk is now configurable! This means my proof of concept document.cookie protection from 2015 might actually work now :)