Question

I am looking into ways to extend Firefox pop-up blocking from an extension. One option is replacing window.open() (or rather Window.prototype.open()) in the webpage by a wrapper function. An important requirement is that this manipulation cannot be detected or reverted by the webpage. For example, if I simply do this:

Window.prototype.open = wrapper;

The webpage can easily revert the change by doing:

delete Window.prototype.open;

Instead I can use Object.defineProperty() to set advanced property flags:

Object.defineProperty(Window.prototype, "open", {value: wrapper, configurable: false});

The webpage can no longer revert this change but it can still detect it: delete Window.prototype.open normally changes the value of Window.prototype.open (different instance of the same function it seems), here delete won’t have any effect at all. Also, Window.prototype.open = "test";delete Window.prototype.open; will produce inconsistent results (different ones depending on whether writable: false flag is specified for the property).

Is there anything else that I can do to emulate the behavior of the original property (short of using binary XPCOM components which has way too many issues of its own)?

Answer 1

In the end I had to give up on using JavaScript proxies for the job. Even though with some effort I can create a wrapper for window.open() that behaves exactly like the original (bug 650299 needs to be considered), there doesn’t seem to be a proper way to replace the original window.open() function. The changed property will always behave differently from the original one, too bad.

So I decided to go with a different approach as a pop-up blocking solution: listen for content-document-global-created notification and have a look at the subject (the new window) as well as its opener. Windows with a non-null opener are pop-up windows of some kind. One can look at the URLs and decide whether the pop-up should be blocked. To block one would call window.stop() (stops all network activities before any network requests are sent) and window.close(). The latter has to be called asynchronously (with a delay) because it will cause a crash otherwise as the initialization of the window continues. Some notes on this approach:

• For pop-ups opening in a new window the window will still show up but disappear immediately. This seems to be unavoidable.
• For the web page it looks like its pop-up window opened but was closed immediately – this isn’t how the built-in pop-up blocker works, more like an external pop-up blocking application.
• New windows always load about:blank first before changing to their actual destination. For same-origin pop-ups the latter won’t send a new content-document-global-created notification which is unfortunate.

All in all: not perfect but usable. And it is very simple, nowhere near the amount of code required for JavaScript proxies.