improve support for default values when using JSON with localstorage

polymer-localstorage.html

@@ -81,15 +81,29 @@
         }
       },
       load: function() {
-        var s = window.localStorage.getItem(this.name);
-        if (s !== null && !this.useRaw) {
-          try {
-            this.value = JSON.parse(s);
-          } catch (x) {
-            this.value = s;
-          }
+        var v = localStorage.getItem(this.name);
+        if (this.useRaw) {
+          this.value = v;
         } else {
-          this.value = s;
+          // localStorage has a flaw that makes it difficult to determine
+          // if a key actually exists or not (getItem returns null if the
+          // key doesn't exist, which is not distinguishable from a stored
+          // null value)
+          // however, if not `useRaw`, an (unparsed) null value unambiguously
+          // signals that there is no value in storage (a stored null value would
+          // be escaped, i.e. "null")
+          // in this case we save any non-null current (default) value
+          if (v === null) {
+            if (this.value !== null) {
+              this.save();
+            }
+          } else {
+            try {
+              v = JSON.parse(v);
+            } catch(x) {
+            }
+            this.value = v;
+          }
         }
         this.loaded = true;
         this.asyncFire('polymer-localstorage-load');
@@ -100,8 +114,8 @@
        * @method save
        */
       save: function() {
-        window.localStorage.setItem(this.name, 
-            this.useRaw ? this.value : JSON.stringify(this.value));
+        var v = this.useRaw ? this.value : JSON.stringify(this.value);
+        localStorage.setItem(this.name, v);
       }
     });
   </script>