Add ability to clear IndexedDB state object store

spec/state-store-spec.js

@@ -21,7 +21,21 @@ describe("StateStore", () => {
     return store.load('no-such-key').then((value) => {
       expect(value).toBeNull()
     })
-  });
+  })
+
+  it("can clear the state object store", () => {
+    const store = new StateStore(databaseName, version)
+    return store.save('key', {foo:'bar'})
+      .then(() => store.count())
+      .then((count) =>
+        expect(count).toBe(1)
+      )
+      .then(() => store.clear())
+      .then(() => store.count())
+      .then((count) => {
+        expect(count).toBe(0)
+      })
+  })
 
   describe("when there is an error reading from the database", () => {
     it("rejects the promise returned by load", () => {

src/state-store.js

@@ -25,9 +25,7 @@ class StateStore {
 
   save (key, value) {
     return this.dbPromise.then(db => {
-      if (!db) {
-        return
-      }
+      if (!db) return
 
       return new Promise((resolve, reject) => {
         var request = db.transaction(['states'], 'readwrite')
@@ -42,9 +40,7 @@ class StateStore {
 
   load (key) {
     return this.dbPromise.then(db => {
-      if (!db) {
-        return null
-      }
+      if (!db) return
 
       return new Promise((resolve, reject) => {
         var request = db.transaction(['states'])
@@ -60,4 +56,36 @@ class StateStore {
       })
     })
   }
+
+  clear () {
+    return this.dbPromise.then(db => {
+      if (!db) return
+
+      return new Promise((resolve, reject) => {
+        var request = db.transaction(['states'], 'readwrite')
+          .objectStore('states')
+          .clear()
+
+        request.onsuccess = resolve
+        request.onerror = reject
+      })
+    })
+  }
+
+  count () {
+    return this.dbPromise.then(db => {
+      if (!db) return
+
+      return new Promise((resolve, reject) => {
+        var request = db.transaction(['states'])
+          .objectStore('states')
+          .count()
+
+        request.onsuccess = () => {
+          resolve(request.result)
+        }
+        request.onerror = reject
+      })
+    })
+  }
 }