How persistent is localStorage?

Solution 1:

Mozilla implements it like cookies:

DOM Storage can be cleared via "Tools -> Clear Recent History -> Cookies" when Time range is "Everything" (via nsICookieManager::removeAll)

https://developer.mozilla.org/en/DOM/Storage

In DOM Storage it is not possible to specify an expiration period for any of your data. All expiration rules are left up to the user. In the case of Mozilla, most of those rules are inherited from the Cookie-related expiration rules. Because of this you can probably expect most of your DOM Storage data to last at least for a meaningful amount of time.

http://ejohn.org/blog/dom-storage/

Chrome implements it like cache:

LocalStorage is Not Secure Storage

HTML5 local storage saves data unencrypted in string form in the regular browser cache.

Persistence

On disk until deleted by user (delete cache) or by the app

https://developers.google.com/web-toolkit/doc/latest/DevGuideHtml5Storage

As for a "replacement for the Cookie", not entirely

Cookies and local storage really serve difference purposes. Cookies are primarily for reading server-side, LocalStorage can only be read client-side. So the question is, in your app, who needs this data — the client or the server?

Solution 2:

Basically, you should not heavily depend on Local Storage.

Local Storage, along with Session Storage, aims to be a replacement of the cookies, defining a more consistent API. There are a few differences from the cookies:

• While the cookies are accessible from both client and server side, Web Storage, in general, and Local Storage, in particular, are accessible only from client side.
• Enhanced capacity (official for cookies is 4 KB) to more than 5MB per domain (Firefox, Google Chrome, and Opera and 10MB in IE).

So yes, your assumption is correct.