[v5]: refactor useMemoSelector

src/react.ts

@@ -25,15 +25,13 @@ const useMemoSelector = <TState, StateSlice>(
   selector: (state: TState) => StateSlice,
 ) =>
   useMemo(() => {
-    let lastSlice: StateSlice
-    let lastState: TState
+    let prev: readonly [TState, StateSlice] | undefined
     return () => {
       const state = getState()
-      if (!Object.is(lastState, state)) {
-        lastSlice = selector(state)
-        lastState = state
+      if (!prev || !Object.is(prev[0], state)) {
+        prev = [state, selector(state)]
       }
-      return lastSlice
+      return prev[1]
     }
   }, [getState, selector])
 

src/react/shallow.ts

@@ -1,4 +1,4 @@
-// import { useDebugValue } from 'react'
+// import { useRef } from 'react'
 // That doesnt work in ESM, because React libs are CJS only.
 // The following is a workaround until ESM is supported.
 // eslint-disable-next-line import/extensions

tests/basic.test.tsx

@@ -680,3 +680,20 @@ it('works with non-object state', async () => {
   fireEvent.click(getByText('button'))
   await findByText('count: 2')
 })
+
+it('works with "undefined" state', async () => {
+  const useUndefined = create(() => undefined)
+
+  const Component = () => {
+    const str = useUndefined((v) => v || 'undefined')
+    return <div>str: {str}</div>
+  }
+
+  const { findByText } = render(
+    <StrictMode>
+      <Component />
+    </StrictMode>,
+  )
+
+  await findByText('str: undefined')
+})