On a browser, sessionStorage in Safari's Priva

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.

Answer 2

Safari latest version (Version 12.0) already have access to sessionStorage without any issue in incognito mode.