feat(types): Make ExtractState public (#2935)

* make ExtractState public and move it to vanilla

* add docs on the matter

* Update vanilla.ts

---------

Co-authored-by: Daishi Kato <dai-shi@users.noreply.github.com>

docs/guides/typescript.md

@@ -161,6 +161,21 @@ It isn't really a lie because `{ bears: number }` is still a subtype of `{ bears
 
 Note that we don't use the curried version when using `combine` because `combine` "creates" the state. When using a middleware that creates the state, it isn't necessary to use the curried version because the state now can be inferred. Another middleware that creates state is `redux`. So when using `combine`, `redux`, or any other custom middleware that creates the state, we don't recommend using the curried version.
 
+If you want to infer state type also outside of state declaration, you can use the `ExtractState` type helper:
+
+```ts
+import { create, ExtractState } from 'zustand'
+import { combine } from 'zustand/middleware'
+
+type BearState = ExtractState<typeof useBearStore>
+
+const useBearStore = create(
+  combine({ bears: 0 }, (set) => ({
+    increase: (by: number) => set((state) => ({ bears: state.bears + by })),
+  })),
+)
+```
+
 ## Using middlewares
 
 You do not have to do anything special to use middlewares in TypeScript.

src/react.ts

@@ -1,14 +1,13 @@
 import React from 'react'
 import { createStore } from './vanilla.ts'
 import type {
+  ExtractState,
   Mutate,
   StateCreator,
   StoreApi,
   StoreMutatorIdentifier,
 } from './vanilla.ts'
 
-type ExtractState<S> = S extends { getState: () => infer T } ? T : never
-
 type ReadonlyStoreApi<T> = Pick<
   StoreApi<T>,
   'getState' | 'getInitialState' | 'subscribe'

src/vanilla.ts

@@ -13,6 +13,8 @@ export interface StoreApi<T> {
   subscribe: (listener: (state: T, prevState: T) => void) => () => void
 }
 
+export type ExtractState<S> = S extends { getState: () => infer T } ? T : never
+
 type Get<T, K, F> = K extends keyof T ? T[K] : F
 
 export type Mutate<S, Ms> = number extends Ms['length' & keyof Ms]