Executing injected Javascript code in a page when developing a Firefox extension

Question

I'm developing a Firefox extension which places a button in the status bar. When the button is clicked, the extension injects some Javascript into the current page. This Javascript has a function that I would like to invoke with some parameters. I've managed injecting the code, I've inspected the page through Firebug and verified that the JS has been injected. How can I call a Javascript function in the page from my extension?

--More information

Here's the code that I'm using to inject my Javascript:

var doc = window.content.document;

//Add the script
var visibilityJS = doc.createElement("script");
visibilityJS.setAttribute("type", "text/javascript");
visibilityJS.setAttribute("charset", "UTF-8");
visibilityJS.setAttribute("src", "chrome://visibility/content/scripts/visibility.js");
head.appendChild(visibilityJS);

//Call the function
alert("Executing testfunction");
window.content.document.defaultView.testFunction();

..and the code inside my JS file that i'm going to inject. i.e. visibility.js

window.testFunction = function() {
    alert("Message");
}

Thanks.

Answer 1

This worked. I don't know the technicalities. I got part of the solution from Felix and part of it from here.

window.content.document.defaultView.wrappedJSObject.testFunction();

Answer 2

If you declare a global variable in your injected code (or explicitly set a property of the window object), then one way do get a reference to this element from your extension, is via the gBrowser object:

gBrowser.contentDocument.defaultView.yourObject
          ^-- HTML document  ^
              object         |-- window object

Be careful though, when you use window and document inside your code. Depending on the context it might refer to the Firefox window or the website window object.