Thanks for the replies guys.
I already knew how to do everything you guys posted, but I just realised I didn't ask my question correctly.
So my issue was, that I'm unable to use the localstorage to save all progress people made in my game, I got hundreds of persistent object and a load of global objects.
So the only reason to save the state of the game was with the save/load system.
Whenever someone updated the game to a new version, but still used a saved state of the previous version, they had issues with the global objects behaving very weird.
The only solution I found for this is to completely wipe all date from the app, with the downside that they lose all saved data/progress.
Thanks to the replies, I did this through events now, by checking the version and just saving an empty savestate over the previous version's savestate if the version is newer.
HOWEVER, this still doesn't solve my initial issue, I need to find an easy way to store the data of persistent and global objects in the Localstorage. So players don't have to start all over again every major update.
I'm currently testing the "nosave" behavior to see if that might help me, but I'm unsure if that works on objects that didn't exist in a previous version.
I know my problem is perfectly achievable with vars/arrays & localstorage, but I would have to do this for every single object, which makes it almost impossible.