Question: Can anybody give some sample code to read and write a file using JavaScript?

Answer: 
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.
You'll have to turn to Flash, Java or Silverlight. In the case of Silverlight, you'll be looking at [Isolated Storage](http://msdn.microsoft.com/en-us/library/3ak841sy(VS.95).aspx). That will get you write to files in your own playground on the users disk. It won't let you write outside of your playground though.