I just created and released backup.js. It's a library to make it easy for html apps to make backups in local storage.
I've been working on and off on my own to do list app. It makes periodic backups of the data in case I accidentally erase something (there's no undo) and in case there's a bug which clobbers data. The backup functionality has been very simple: it makes copies of data in local storage but never cleans them up. Eventually it hits the browser's local storage space limit and I have to dig in and delete some old backups to make the app start working again.
I finally got sick of this situation and decided to implement backups in a smarter way. backup.js is a standalone library to do this. I easily plugged it into my to do list app, but it's perfectly usable for any future projects of mine or anyone else who wants to use it.
It's been a fun little side project for a few days!