fix(vanilla): unexpected null state update behavior (#2213)

* fix: unexpected null state update behavior

* chore: lint code

* test: add test for setting state to null

---------

Co-authored-by: wulimao <tao@trlab.com>

src/vanilla.ts

@@ -73,7 +73,7 @@ const createStoreImpl: CreateStoreImpl = (createState) => {
     if (!Object.is(nextState, state)) {
       const previousState = state
       state =
-        replace ?? typeof nextState !== 'object'
+        replace ?? (typeof nextState !== 'object' || nextState === null)
           ? (nextState as TState)
           : Object.assign({}, state, nextState)
       listeners.forEach((listener) => listener(state, previousState))

tests/vanilla/basic.test.ts

@@ -112,6 +112,24 @@ it('can set the store without merging', () => {
   expect(getState()).toEqual({ b: 2 })
 })
 
+it('can set the object store to null', () => {
+  const { setState, getState } = createStore<{ a: number } | null>(() => ({
+    a: 1,
+  }))
+
+  setState(null)
+
+  expect(getState()).toEqual(null)
+})
+
+it('can set the non-object store to null', () => {
+  const { setState, getState } = createStore<string | null>(() => 'value')
+
+  setState(null)
+
+  expect(getState()).toEqual(null)
+})
+
 it('works with non-object state', () => {
   const store = createStore<number>(() => 1)
   const inc = () => store.setState((c) => c + 1)