app-session.html

<link rel="import" href="../polymer/polymer.html">
<script>
  Polymer({

    is: 'app-session',

    properties: {
      prefix: {
        type: String,
        value: 'session_',
        readOnly: true
      },
      storage: {
        type: Object,
        value: function () {
          return window.localStorage;
        }
      }
    },

    has: function (key) {
      var prefixed_key = this.prefix + key;
      var value = this.storage.getItem(prefixed_key);
      return value !== undefined;
    },

    get: function (key) {
      var prefixed_key = this.prefix + key;
      var value = this.storage.getItem(prefixed_key);
      var parsed;
      try {
        parsed = JSON.parse(value);
      } catch (e) {
        parsed = null;
      }
      return parsed;
    },

    set: function (key, value) {
      var prefixed_key = this.prefix + key;
      var stringfied = JSON.stringify(value);
      this.storage.setItem(prefixed_key, stringfied);
    },

    unset: function (key) {
      var prefixed_key = this.prefix + key;
      this.storage.removeItem(prefixed_key);
    },

    clear: function () {
      this.storage.clear();
    }

  });
</script>