How to read and write into file using JavaScript?
For completeness, the OP does not state he is looking to do this in a browser (if he is, as has been stated, it is generally not possible)
However javascript per se does allow this; it can be done with server side javascript.
See this documentation on the Javascript File class
Edit: That link was to the Sun docs that now have been moved by Oracle.
To keep up with the times here's the node.js documentation for the FileSystem class: http://nodejs.org/docs/latest/api/fs.html
Edit(2): You can read files client side now with HTML5: http://www.html5rocks.com/en/tutorials/file/dndfiles/
No. Browser-side javascript doesn't have permission to write to the client machine without a lot of security options having to be disabled
The future is here! The proposals are closer to completion, no more ActiveX or flash or java. Now we can use:
You could use the Drag/Drop to get the file into the browser, or a simple upload control. Once the user has selected a file, you can read it w/ Javascript: http://www.html5rocks.com/en/tutorials/file/dndfiles/