Поправил тесты

src/effect.test.ts

@@ -104,14 +104,11 @@ describe('createEffect', () => {
         })
     );
 
-    fetchApiFx.pending.watch(watcherMock);
+    fetchApiFx.pending.watch((value) => watcherMock(value));
 
     await fetchApiFx(100);
 
-    expect(watcherMock.mock.calls).toEqual([
-      [true, true],
-      [false, false],
-    ]);
+    expect(watcherMock.mock.calls).toEqual([[true], [false]]);
   });
 
   it('`.done/doneData`: события, которое срабатывают с результатом выполнения эффекта', async () => {

src/store.test.ts

@@ -22,37 +22,51 @@ describe('createStore', () => {
     expect($store.getState()).toBe(1);
   });
 
+  it('Не обновлять стор если редьюсер возвращает `undefined`', () => {
+    const eventMock = jest.fn(<T>() => createEvent<T>())();
+    const $store = createStore(0).on(eventMock, (state) => state);
+
+    expect($store.getState()).toBe(0);
+
+    eventMock(undefined);
+
+    expect($store.getState()).toBe(0);
+  });
+
   it('`.watch`: вызывает функцию с сайд-эффектами при каждом обновлении стора', () => {
     const watcherMock = jest.fn();
+    const triggerWatcherMock = jest.fn();
     const add = createEvent<number>();
     const foo = createEvent<number>();
     const $store = createStore(0).on(add, (state, payload) => state + payload);
 
-    $store.watch(watcherMock);
-    $store.watch(foo, watcherMock);
+    $store.watch((value) => watcherMock(value));
+    $store.watch(foo, triggerWatcherMock);
 
     add(4);
-    expect(watcherMock).toHaveBeenCalledWith(4, 4);
+    expect(watcherMock).toHaveBeenCalledWith(4);
+    expect(triggerWatcherMock).not.toHaveBeenCalled();
     expect($store.getState()).toBe(4);
 
     add(3);
-    expect(watcherMock).toHaveBeenCalledWith(7, 3);
+    expect(watcherMock).toHaveBeenCalledWith(7);
+    expect(triggerWatcherMock).not.toHaveBeenCalled();
     expect($store.getState()).toBe(7);
 
     foo(42);
-    expect(watcherMock).toHaveBeenCalledWith(7, 42);
-    expect($store.getState()).toBe(7);
+    expect(triggerWatcherMock).toHaveBeenCalledWith(7, 42);
   });
 
   it('`unwatch` прекращает отслеживание', () => {
     const watcherMock = jest.fn();
     const add = createEvent<number>();
     const $store = createStore(0).on(add, (state, payload) => state + payload);
 
-    const unwatch = $store.watch(watcherMock);
+    const unwatch = $store.watch((value) => watcherMock(value));
 
     add(42);
-    expect(watcherMock).toHaveBeenCalledWith(42, 42);
+    expect(watcherMock).toHaveBeenCalledWith(42);
+    expect($store.getState()).toBe(42);
 
     unwatch();
     add(123);
@@ -65,17 +79,17 @@ describe('createStore', () => {
     const changed = createEvent<string>();
     const $title = createStore('').on(changed, (_, newTitle) => newTitle);
 
-    $title.map((title) => title.length).watch(watcherMock);
+    $title.map((title) => title.length).watch((length) => watcherMock(length));
 
     changed('hello');
-    expect(watcherMock).toHaveBeenCalledWith(5, 5);
+    expect(watcherMock).toHaveBeenCalledWith(5);
     expect(watcherMock).toHaveBeenCalledTimes(1);
 
     changed('world');
     expect(watcherMock).toHaveBeenCalledTimes(1);
 
     changed('hello world');
-    expect(watcherMock).toHaveBeenCalledWith(11, 11);
+    expect(watcherMock).toHaveBeenCalledWith(11);
     expect(watcherMock).toHaveBeenCalledTimes(2);
   });
 

src/store.ts

@@ -27,6 +27,8 @@ export function createStore<State>(defaultState: State): Store<State> {
   let state = defaultState;
 
   function update<Payload>(newState: State, payload: Payload): void {
+    if (newState === undefined || newState === state) return;
+
     state = newState;
     watchers.forEach((watcher) => watcher(state, payload));
   }