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)?

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)?

• javascript
• firefox
• firefox-addon
• firefox4
• prototype-programming

• Why does it have to be undetectable? – cwallenpoole Commented Jul 11, 2011 at 18:24
• 1 @cwallenpoole: The webpage shouldn't know that anything beyond the usual pop-up blocker is being used - some webmasters tend to use some very unfriendly techniques if they get a chance. – Wladimir Palant Commented Jul 11, 2011 at 18:30
• Are you ruling out use of a binary XPCOM component? – Matthew Gertner Commented Jul 12, 2011 at 12:10
• 1 @Matthew: No, using binary XPCOM components definitely isn't going to work (see adblockplus.org/blog/…). – Wladimir Palant Commented Jul 12, 2011 at 12:56

4 Answers 4

You might try using the nsIWindowWatcher interface to register your own window creator (nsIWindowCreator). That way you can control whether a new window is opened without affecting the window object itself (and thus remaining invisible to web sites).

I'm not sure whether the inability to change the implementation of window.open() without this being detectable is a bug. Perhaps it's just not considered an important requirement for methods like Object.defineProperty. But it might be worth filing a bug to see what others think about making this an option in the future. After all, ad blocking is a major use case.

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.

Web browsers intentionally prevent this behavior, it's for maintaing the security of web e.g. when you use iFrame you don't want that iFrame to mess up or hack your page.

But instead of manipulating the window object properties why not to create a wrapper for the window object and override window by the wrapper locally?

Example:

// Copy window object to wraper
var wrapper = {};
for(prop in window) {
  wrapper[prop] = window[prop];
}

wrapper.open = function yourNewOpenFunction() {
  /// do your custom code here
}

(function fakeScope(window){
    window.open(); // this is wrapper.open
}(wrapper));

BTW this affects only the body inside fakeScope() function, and cannot be applied globally.

it striked me this morning: you can use Object.freeze(Window.prototype); ! test have shown, that methods protected with this cannon be deleted, but they can be easily detected.

old answer:

What about ES:Harmony Proxies ? http://brendaneich.com/2010/11/proxy-inception/

Of course, they are unstable, but they are working in Firefox 4+, and you are not the man, who is afraid of difficulties ;)