create and modify xml file using javascript

I've found Ariel Flesler's XMLWriter constructor function to be a good start for creating XML from scratch, take a look at this

http://flesler.blogspot.com/2008/03/xmlwriter-for-javascript.html

Example

function test(){    
   var v = new  XMLWriter();
   v.writeStartDocument(true);
   v.writeElementString('test','Hello World');
   v.writeAttributeString('foo','bar');
   v.writeEndDocument();
   console.log( v.flush() );
}

Result

<?xml version="1.0" encoding="ISO-8859-1" standalone="true" ?>
<test foo="bar">Hello World</test>

One caveat to keep in mind is that it doesn't escape strings.

See also Libraries to write xml with JavaScript

In IE you can manipulate XML using an ActiveX.
There is also a built in object for FF and other W3C complient browsers.
I recommend you to take a look at this article.

What context is the file running in, and where are you going to save the new XML data?

(The usual context is the browser, in which case you're basically able to display it or post it back to the server.)

But if you're writing a script that would run outside the browser, it depends.

AFAIK you can't really create a new xml file with client-side Javascript, usually this is done through a server-side language ( Javascript can be used for server side but it's a rarity ).

Is there any particular reason why you can't go with PHP or Python to create the xml file?