Or maybe return window.localStorage != null, which would weed out both — null and undefined (and so not declared too).

we have the in test as Firefox throws similar fits when cookies are disabled..

from the source
// both localStorage and sessionStorage are
// tested in this method because otherwise Firefox will
// throw an error: https://bugzilla.mozilla.org/show_bug.cgi?id=365772
// if cookies are disabled

Maybe a test in advance for disabled cookies then ?
Write and re-read the same cookie, if cookies are disabled no error is thrown.

localstorage test failing in IE when it was disabled. thx kangax. closed by d8947c9. other optimizations