localStorageWrapper that silently fails (reports error to console) on write fail. This is one of the options to tackle Safari's zero quota of localStorage when in private mode.

localStorageWrapper.js

  /**
   * A constructor for a wrapper class implementing the localStorage public interface
   * (getItem, setItem, removeItem, key, clear) and some of our extensions
   * (getObject, setObject, removeObject, getObjectEvenIfNull).
   *
   * The idea is to create global `localStorageWrapper` and `sessionStorageWrapper` variables,
   * to be used instead of `localStorage` and `sessionStorage` (see below).
   * This is because in Safari on iOS in private mode, localStorage has a quota of 0
   * and always throws an exception when trying to save anything.
   *
   * Instead of wrapping all localStorage writes with a try-catch, it's better to
   * use a wrapper class instead and silently fail when we can not save.
   *
   * Note that `localStorageWrapper` does not implement `.length` property of localStorage
   * and instead exposes `.getLength()` method (we could have implemented `.length` using
   * Object.defineProperty or get syntax, but this is not supported in IE8-).
   *
   * @param {Storage} underlyingStorage either localStorage or sessionStorage
   * @return Object prototype of a wrapper class that passes the calls to underlyingStorage
   *   and fails silently on errors
   */
  var StorageWrapperProto = function(underlyingStorage) {
    return {
      /////////////////////////////// WRAPPER OF NATIVE LOCALSTORAGE METHODS//////////////////////////////
      key: function(n) {
        return underlyingStorage.key(n);
      },

      clear: function() {
        underlyingStorage.clear();
      },

      getItem: function(key) {
        return underlyingStorage.getItem(key);
      },

      setItem: function(key, value) {
        try {
          underlyingStorage.setItem(key, value);
        } catch (e) {
          window.console.error("Error writing to the storage for key = " + key +
            "! (this is expected in private mode in Safari)");
        }
      },

      removeItem: function(key) {
        underlyingStorage.removeItem(key);
      },

      /////////////////////////////// BELOW ARE OUR CURRENT EXTENSIONS ///////////////////////////////////
      getLength: function() {
        return underlyingStorage.length;
      },

      removeObject: function(key) {
        this.removeItem(key);
      },

      setObject: function(key, value) {
        if (value !== null) {
          var json = JSON.stringify(value);
          this.setItem(key, json);
        }
      },

      getObject: function(key) {
        var json = null;
        json = this.getItem(key);
        if (!isUndefined(json) && json != "") {
          json = JSON.parse(json);
        }
        return json;
      },

      /*
       * This is to be used when we don't care if something is null in local storage,
       * we always want to have an object to store properties on it (this is to be used
       * in code paths where we assumed that we will always retrieve proper thing from
       * local storage, like user profile, while since Safari in private mode can not
       * store anything, it always returns null entries whatever we ask for)
       */
      getObjectEvenIfNull: function(key) {
        return this.getObject(key) || {};
      }
    };
  };

  // this requires `Object.create`; use polyfill if need to support old IEs
  window.localStorageWrapper = Object.create(new StorageWrapperProto(window.localStorage));
  window.sessionStorageWrapper = Object.create(new StorageWrapperProto(window.sessionStorage));