experimental: unstable ssrSafe middleware (#3308)

* unstable ssrSafe

* add todo comment

Author: dai-shi

src/middleware.ts

@@ -14,3 +14,4 @@ export {
   type PersistStorage,
   type PersistOptions,
 } from './middleware/persist.ts'
+export { ssrSafe as unstable_ssrSafe } from './middleware/ssrSafe.ts'

src/middleware/ssrSafe.ts

@@ -0,0 +1,25 @@
+import type { StateCreator, StoreMutatorIdentifier } from '../vanilla.ts'
+
+// This is experimental middleware. It will be changed before finalizing it.
+// https://github.com/pmndrs/zustand/discussions/2740
+// TODO Not very happy with the middleware name. Will revisit it later.
+export function ssrSafe<
+  T extends object,
+  U extends object,
+  Mps extends [StoreMutatorIdentifier, unknown][] = [],
+  Mcs extends [StoreMutatorIdentifier, unknown][] = [],
+>(
+  config: StateCreator<T, Mps, Mcs, U>,
+  isSSR: boolean = typeof window === 'undefined',
+): StateCreator<T, Mps, Mcs, U> {
+  return (set, get, api) => {
+    if (!isSSR) {
+      return config(set, get, api)
+    }
+    const ssrSet = () => {
+      throw new Error('Cannot set state of Zustand store in SSR')
+    }
+    api.setState = ssrSet
+    return config(ssrSet as never, get, api)
+  }
+}