TiddlyWiki is pure HTML and JavaScript 1 page wiki, but it allows the user to save content back to the HTML file. Does anyone know how this is achieved?

http://www.tiddlywiki.com

I haven't had time to look at the code (there appears to be a lot), so I'm hoping someone here knows the answer and saves me hours of code review. My guess is it is creating a client side cache; i.e. data in a cookie, etc... but I'm hoping its actually modifying the HTML, but I'm guessing this isn't feasible as JavaScript is client side (excluding serverside JS stuff like node).

Any help would be welcomed.