Modify JavaScript code when a page is load

[Paul Saint Leger Morales]

Hi,
            Is it possible to modify JavaScript code when a web page
is loading? I have been proving the followings techniques:

*  
http://groups.google.com/group/mozilla.dev.tech.network/browse_thread/thread/62af84e6d35ab77d/df5a79e36a4caf24#df5a79e36a4caf24

These techniques in nutshell is to use an "observer" and "converter"
objects. Using "observer" object i modify the MIME type of document
and using the "converter" object i modify the document.

            However, i have not attained to modify the JavaScript
code, only i have attained to modify HTML to HTML and WML to HTML
(http://wmlbrowser.mozdev.org/index.html).

Anybody can give a help, please.

Here i write an extract of my code (maybe, i am doing bad something):

//This function modify a document.
JavaScriptConverter.prototype.onStopRequest =
    function (aRequest, aContext, aStatusCode) {

        var converter = Components
        .classes["@mozilla.org/intl/scriptableunicodeconverter"]
        .createInstance
(Components.interfaces.nsIScriptableUnicodeConverter);

        converter.charset = this.charset;

        try {
            this.data = converter.ConvertToUnicode
(this.unencodeddata);
        } catch (failure) {
            this.logger.logStringMessage ("wmlbrowser: Failure
converting unicode using charset " + this.charset);
            this.logger.logStringMessage (failure);
            this.data += this.unencodeddata;
        }

        var wrapData = rewrite(this.data);

      var sis =
        Components.classes["@mozilla.org/io/string-input-stream;1"]
        .createInstance(Components.interfaces.nsIStringInputStream);
         sis.setData (wrapData, wrapData.length);

        // Pass the data to the main content listener
        this.listener.onDataAvailable (this.channel, aContext, sis, 0,
wrapData.length);

        this.listener.onStopRequest (this.channel, aContext,
aStatusCode);
    };


Observer.prototype = {
    observe: function(subject,topic,data){
        subject.QueryInterface(Components.interfaces.nsIChannel);
        myDump("Observer:prototype:1:"+subject.contentType+"
data:"+data+ " topic:"+topic);

        if (subject.contentType == "text/javascript" ||
subject.contentType == "application/x-javascript")
            subject.contentType = "text/vnd.wap.wml";

        contenido = subject.contentType;

        myDump("Observer:prototype:2:"+subject.contentType+"
data:"+data+" topic:"+topic);
    }
.....
}


Thank in advance,
Paul Leger.
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