isWebStorageSupported.js

;(function(window, undefined) {
  var checkKey = '__WEB_STORAGE_CHECK__';

  var isSupported = function(localOrSession) {
    var _storage = window[localOrSession];
    return _storage ? (function() {
      try {
        _storage.setItem(checkKey, 1);
        _storage.getItem(checkKey);
        _storage.removeItem(checkKey);
        return true;
      } catch (e) {
        return false;
      }
    })() : false;
  };

  // 参照しやすいようにnavigatorオブジェクトを拡張する
  navigator.isLocalStorageSupported = isSupported('localStorage');
  navigator.isSessionStorageSupported = isSupported('sessionStorage');
})(this);