html5 localStorage error with Safari: "QUOTA_EXCEEDED_ERR: DOM Exception 22: An attempt was made to add something to storage that exceeded the quota."

Apparently this is by design. When Safari (OS X or iOS) is in private browsing mode, it appears as though localStorage is available, but trying to call setItem throws an exception.

store.js line 73
"QUOTA_EXCEEDED_ERR: DOM Exception 22: An attempt was made to add something to storage that exceeded the quota."

What happens is that the window object still exposes localStorage in the global namespace, but when you call setItem, this exception is thrown. Any calls to removeItem are ignored.

I believe the simplest fix (although I haven't tested this cross browser yet) would be to alter the function isLocalStorageNameSupported() to test that you can also set some value.

https://github.com/marcuswestin/store.js/issues/42

function isLocalStorageNameSupported() 
{
    var testKey = 'test', storage = window.sessionStorage;
    try 
    {
        storage.setItem(testKey, '1');
        storage.removeItem(testKey);
        return localStorageName in win && win[localStorageName];
    } 
    catch (error) 
    {
        return false;
    }
}

The fix posted on above link did not work for me. This did:

function isLocalStorageNameSupported() {
  var testKey = 'test', storage = window.localStorage;
  try {
    storage.setItem(testKey, '1');
    storage.removeItem(testKey);
    return true;
  } catch (error) {
    return false;
  }
}

Derived from http://m.cg/post/13095478393/detect-private-browsing-mode-in-mobile-safari-on-ios5

As mentioned in other answers, you'll always get the QuotaExceededError in Safari Private Browser Mode on both iOS and OS X when localStorage.setItem (or sessionStorage.setItem) is called.

One solution is to do a try/catch or Modernizr check in each instance of using setItem.

However if you want a shim that simply globally stops this error being thrown, to prevent the rest of your JavaScript from breaking, you can use this:

https://gist.github.com/philfreo/68ea3cd980d72383c951

// Safari, in Private Browsing Mode, looks like it supports localStorage but all calls to setItem
// throw QuotaExceededError. We're going to detect this and just silently drop any calls to setItem
// to avoid the entire page breaking, without having to do a check at each usage of Storage.
if (typeof localStorage === 'object') {
    try {
        localStorage.setItem('localStorage', 1);
        localStorage.removeItem('localStorage');
    } catch (e) {
        Storage.prototype._setItem = Storage.prototype.setItem;
        Storage.prototype.setItem = function() {};
        alert('Your web browser does not support storing settings locally. In Safari, the most common cause of this is using "Private Browsing Mode". Some settings may not save or some features may not work properly for you.');
    }
}