On a browser, sessionStorage in Safari's Private Browsing does not work the same as Chrome's Incognito Mode and Firefox's Private Window?

Question

It seems that for sessionStorage, it works differently on Chrome's Incognito Mode vs Safari's Private Browsing and Firefox's Private Window? I can find something on http://www.webdirections.org/blog/webstorage-persistent-client-side-data-storage/ but it doesn't say that Safari's Private Browsing will throw an exception.

The following is how I opened "Private Browsing":

1. On Safari on Mac, click "Safari -> Private Browsing" on the menu bar
2. On Chrome, use "File -> New Incognito Window"
3. On Firefox, use "File -> New Private Window"

and on Safari, sessionStorage does not work, and if I do the following in the console:

> sessionStorage["foo"] = 123.4 Error: QUOTA_EXCEEDED_ERR: DOM Exception 22  > sessionStorage["foo"]  undefined 

but on Chrome or Firefox, sessionStorage works as usual (as non-private browsing). Is the above accurate as far as sessionStorage is concerned?

Answer 1

Your assessment is practically accurate:

• Safari will just use a quota of 0 in private mode, so all attempts to set a value will fail. This is kinda OK according to the spec, as the spec does not mandate a minimum space requirement.
• Chrome and Firefox still allow you to use storage, however private storage is independent from non-private, i.e. setting an item in private mode will not reflect back into non-private mode (important for localStorage only).

Please note that other browsers are also free to throw QuotaExceededError exceptions at any given time, should you go over the quota.