As I pointed out below, Amplify.js, an established Javascript library, has the Store module that supports all manner of browser storage (including userData) with a unified interface that abstracts away the specific storage mechanism.

http://amplifyjs.com/api/store/

I have nothing to do with that project but I hate to see people re-invent the wheel. ;-)

Simply because I didn't know about it. Happy look at pull requests or I'll have a go at it myself if I get time.