[dotproto]

I was about to say "concurrent writes shouldn't be a problem because localStorage is synchronous and JS is single-threaded," but then I started thinking about multiple tabs, WebWorkers, and multi-process browsers and figured I should double-check the spec.

> Warning! The localStorage getter provides access to shared state. This specification does not define the interaction with other agent clusters in a multiprocess user agent, and authors are encouraged to assume that there is no locking mechanism. A site could, for instance, try to read the value of a key, increment its value, then write it back out, using the new value as a unique identifier for the session; if the site does this twice in two different browser windows at the same time, it might end up using the same "unique" identifier for both sessions, with potentially disastrous effects.

https://html.spec.whatwg.org/multipage/webstorage.html#intro...