Many people assume that client-side JavaScript cannot perform file-system operations on the client computer. Actually, the objects needed are often found in the browsers, and the rest is only a matter of compatibility (the way IE saves files is very different from the way Firefox saves files...) and of permissions.

In order to perform critical file-system operations, the script must be granted privileges. There are different ways to get these privileges, and unfortunately, these are very different depending on the browser your application run on.

I won't debate long on the subject of security and permissions in the browser, others did that much better than me (check Google for more information). The demo here demonstrates as a proof of concept that saving a file locally is possible in both major web clients available today (IE and Firefox, tested only on Windows platform).