###Run
npm install && npm start then in the browser visit default url http://localhost:3000 for React version or append ?vanilla (http://localhost:3000?vanilla) to get vanilla JS version.
Both (React and vanilla) versions do the same thing: load list of emails on "load" button click and mark given email as read by clicking on it.
Now, in React version emailsAsList derivation calculation is called multiple times (way too many I guess) even on single click, while on vanilla version it works as expected (called once at first - empty list, then when emails get loaded and later on every email click).
Shouldn't derivation calculations be re-used if not changed in react version too?