I tried to use something like document.forms in Firefox-addon script but it doesn't work.

So, I need to manipulate DOM objects in Firefox-addon script such as forms, inputs... etc. How can I do that without using SDK?

I tried to use something like document.forms in Firefox-addon script but it doesn't work.

So, I need to manipulate DOM objects in Firefox-addon script such as forms, inputs... etc. How can I do that without using SDK?

• javascript
• firefox-addon
• xul

• Where is your script running? Or how are you invoking it? – Wladimir Palant Commented Jul 8, 2014 at 8:06
• It is a javascript file inside my Firefox plugin ".xpi" file. – user3377708 Commented Jul 8, 2014 at 10:47
• A JavaScript file inside an xpi file won't execute just like that. Once again, how are you running it? Browser overlay? XPCOM ponent? SDK module? – Wladimir Palant Commented Jul 8, 2014 at 11:14
• Yes Browser overlay ".xul" file. – user3377708 Commented Jul 8, 2014 at 11:18

2 Answers 2

document.forms will not work, because document is not what you think it is: It is the top level browser (Firefox) window, and not the content in a tab.

A Firefox browser window can have multiple tabs, one of which is the active tab. The active tab <browser> element (which is the XUL element containing the actual content document) also has a shortcut named content, e.g. content.document.forms will be a collection of forms in the active tab. So you'll have to adjust your mental model here from

window and document refer to a website

to

window and document refer to the top-level browser window that may contain a lot of different websites.

The top-level window is more like a document containing multiple frames (the actual websites), really, but with a different APIs to access them.

So, e.g. when executing some action after the user pressed some add-on toolbar button, it might be enough to just use content.document.forms to get the forms of the currently active tab.

But using content. is often not enough: Add-ons would listen for page loads in tabs as the user navigates by adding appropriate event listeners to the <tabbrowser> element (gBrowser), which is the element containing all tabs. MDN has some code snippets for that and lots of other stuff.

Other add-ons add item(s) to the content context menu (contentAreaContextMenu) and use the popupshowing event to know what DOM node (and by this what .ownerDocument and content window == .ownerDocument.defaultView) is currently focused.

An important thing to always keep in mind: Your add-on code runs with full privileges, while websites of course do not. So be careful not to write insecure code. E.g. all forms of unbound eval are evil.

Judging by your ments, your code is running in the context of the browser window. This means that document refers to the document of the browser window, not the document that is loaded into it. The easiest way to get to the latter is using the window.content property:

var contentDoc = content.document;
alert(contentDoc.forms.length);

This will give you only the current tab however. For the other tabs you can use the APIs provided by the <tabbrowser> element (accessible via the global gBrowser variable), e.g. to access the first tab:

var contentDoc = gBrowser.browsers[0].contentDocument;
alert(contentDoc.forms.length);