Do not assign if returning undefined

CHANGELOG.md

@@ -1,5 +1,13 @@
 # Changelog
 
+## 9.8.0
+
+### Noticeable changes
+
+- If a store returns undefined, no state change will occur. This is
+  potentially a breaking change. If you have stores that return
+  `undefined`, consider changing your logic to support returning `null`
+
 ## 9.7.0
 
 - The third argument of store callbacks now contains all application

src/Microcosm.js

@@ -101,8 +101,11 @@ Microcosm.prototype = {
     for (var i = 0; i < this.stores.length; i++) {
       var key   = this.stores[i][0]
       var store = this.stores[i][1]
+      var answer = send(store, key, state, transaction)
 
-      next[key] = send(store, key, state, transaction)
+      if (answer !== void 0) {
+        next[key] = answer
+      }
     }
 
     return next

src/__tests__/serialization-test.js

@@ -40,6 +40,18 @@ describe('Serialization', function() {
     assert.equal(false, 'fiz' in app.deserialize(undefined))
   })
 
+  it ('defaults to getInitialState when no deserialize method is provided', function() {
+    let app = new Microcosm()
+
+    app.addStore('fiz', {
+      getInitialState() {
+        return true
+      }
+    })
+
+    assert.equal(true, app.replace({}).state.fiz)
+  })
+
   it ('passes the raw data as the seconda argument of deserialize', function(done) {
     let app = new Microcosm()