Javascript alone doesn't have any way to access the local computer's file system for WRITE purposes. Period.
However, Downloadify, by Doug Neiner, was built for this purpose and uses a combination of Javascript and the Flash library.
It needs Flash 10 to work.
Alternately, you can install apache onto the computer (or better yet, a full stack like XAMPP or WAMP/MAMP/LAMP) and use PHP (with javascript/ajax) to write files onto the local file system. However, this means that the website must also be hosted locally. Probably your best bet is Downloadify
Resources:
https://github.com/dcneiner/Downloadify
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