sessionStorage in old versions of Internet Explorer

I'm trying to use sessionStorage to display a modal alert only once per session - and I'm trying to do this in IE versions < 9.

The alert will tell users how terrible their IE version < 9 is, and to please upgrade.

Using sessionStorage works fine in IE 11. But in older versions of IE, I can't get it to take effect (sessionStorage comes back as 'undefined or null'). I've tested this locally and on a live site. Does sessionStorage even work in IE < 9, and if not, what's a good alternative I can use in these older IE versions to only display this alert once per 'session'?

My code looks like:

if (sessionStorage.getItem('browserCheck') !== 'true') {
     // define alert function
     // call alert function
    sessionStorage.setItem('browserCheck','true');
}

So apparently, sessionStorage is valid in IE 7 and 8. I was just deleting my cache wrong in the browser before switching the dev tools emulator from IE11 to IE 7 or 8. I was able to fix this by unchecking the top box inside of the 'delete history' prompt in IE:

Earlier, sessionStorage was coming back as 'undefined or null' in IE 7 and 8 in my dev tools console - but I think this was a because of a separate issue. It isn't undefined - it works.

I believe localStorage actually is invalid in IE < 8 though - so if localStorage is your problem, this link is helpful: HTML5 Local Storage fallback solutions