Convert `localStorage` service to JS

src/sidebar/local-storage.coffee

-module.exports = ['$window', ($window) ->
-  # Detection is needed because we run often as a third party widget and
-  # third party storage blocking often blocks cookies and local storage
-  # https://github.com/Modernizr/Modernizr/blob/master/feature-detects/storage/localstorage.js
-  storage = do ->
-    key = 'hypothesis.testKey'
-    try
-      $window.localStorage.setItem  key, key
-      $window.localStorage.removeItem key
-      $window.localStorage
-    catch
-      memoryStorage = {}
-      getItem: (key) ->
-        if key of memoryStorage then memoryStorage[key] else null
-      setItem: (key, value) ->
-        memoryStorage[key] = value
-      removeItem: (key) ->
-        delete memoryStorage[key]
-
-  return {
-    getItem: (key) ->
-      storage.getItem key
-    getObject: (key) ->
-      json = storage.getItem key
-      return JSON.parse json if json
-      null
-    setItem: (key, value) ->
-      storage.setItem key, value
-    setObject: (key, value) ->
-      repr = JSON.stringify value
-      storage.setItem key, repr
-    removeItem: (key) ->
-      storage.removeItem key
-  }
-]

src/sidebar/local-storage.js

+'use strict';
+
+/**
+ * Fallback in-memory store if `localStorage` is not read/writable.
+ */
+class InMemoryStorage {
+  constructor() {
+    this._store = {};
+  }
+
+  getItem(key) {
+    return (key in this._store) ? this._store[key] : null;
+  }
+
+  setItem(key, value) {
+    this._store[key] = value;
+  }
+
+  removeItem(key) {
+    delete this._store[key];
+  }
+}
+
+/**
+ * A wrapper around the `localStorage` API which provides a fallback to
+ * in-memory storage in browsers that block access to `window.localStorage`.
+ * in third-party iframes.
+ */
+// @ngInject
+function localStorage($window) {
+  var storage = $window.localStorage;
+
+  try {
+    // Test whether we can read/write localStorage.
+    var key = 'hypothesis.testKey';
+    $window.localStorage.setItem(key, key);
+    $window.localStorage.getItem(key);
+    $window.localStorage.removeItem(key);
+  } catch (e) {
+    storage = new InMemoryStorage();
+  }
+
+  return {
+    getItem(key) {
+      return storage.getItem(key);
+    },
+
+    getObject(key) {
+      var item = storage.getItem(key);
+      return item ? JSON.parse(item) : null;
+    },
+
+    setItem(key, value) {
+      storage.setItem(key, value);
+    },
+
+    setObject(key, value) {
+      var repr = JSON.stringify(value);
+      storage.setItem(key, repr);
+    },
+
+    removeItem(key) {
+      storage.removeItem(key);
+    },
+  };
+}
+
+module.exports = localStorage;