Can anybody give some sample code to read and write a file using JavaScript?
here's the mozilla proposal
http://www-archive.mozilla.org/js/js-file-object.html
this is implemented with a compilation switch in spidermonkey, and also in adobe's extendscript. Additionally (I think) you get the File object in firefox extensions.
rhino has a (rather rudementary) readFile function https://developer.mozilla.org/en/Rhino_Shell
for more complex file operations in rhino, you can use java.io.File methods.
you won't get any of this stuff in the browser though. For similar functionality in a browser you can use the SQL database functions from HTML5, clientside persistence, cookies, and flash storage objects.