#2808 Preloaded state is now selectively partial (instead of deeply partial). (#3485)

* Preloaded state is now selectively partial (instead of deeply partial).

* Improved CombinedState, PreloadedState, and removed UnCombinedState.

Found a better way to type check CombinedState which allows the
$CombinedState symbol property marker to be optional. Since it's
optional, it's no longer necessary to strip it off in the Reducer state
parameter type and return type. This leaves the type definition for
Reducer unmodified, reduces the number of types required by one, and
makes the resolved types and stack traces clearer.

* Small change to the description of CombinedState.

* Removed DeepPartial import from tests.

Leaving the definition in place as removing it would be a breaking change.

* Made prettier happy.

* Made prettier happy with UsingObjectSpreadOperator.md

Author: Shakeskeyboarde

docs/recipes/UsingObjectSpreadOperator.md

@@ -64,6 +64,7 @@ While the object spread syntax is a [Stage 4](https://github.com/tc39/proposal-o
   "plugins": ["@babel/plugin-proposal-object-rest-spread"]
 }
 ```
+
 > ##### Note on Object Spread Operator
 
-> Like the Array Spread Operator, the Object Spread Operator creates a [shallow clone](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax#Spread_in_object_literals) of the original object. In other words, for multidimensional source objects, elements in the copied object at a depth greater than one are mere references to the source object (with the exception of [primitives](https://developer.mozilla.org/en-US/docs/Glossary/Primitive), which are copied). Thus, you cannot reliably use the  Object Spread Operator (`...`) for deep cloning objects.
+> Like the Array Spread Operator, the Object Spread Operator creates a [shallow clone](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax#Spread_in_object_literals) of the original object. In other words, for multidimensional source objects, elements in the copied object at a depth greater than one are mere references to the source object (with the exception of [primitives](https://developer.mozilla.org/en-US/docs/Glossary/Primitive), which are copied). Thus, you cannot reliably use the Object Spread Operator (`...`) for deep cloning objects.

index.d.ts

@@ -32,6 +32,46 @@ export interface AnyAction extends Action {
   [extraProps: string]: any
 }
 
+/**
+ * Internal "virtual" symbol used to make the `CombinedState` type unique.
+ */
+declare const $CombinedState: unique symbol
+
+/**
+ * State base type for reducers created with `combineReducers()`.
+ *
+ * This type allows the `createStore()` method to infer which levels of the
+ * preloaded state can be partial.
+ *
+ * Because Typescript is really duck-typed, a type needs to have some
+ * identifying property to differentiate it from other types with matching
+ * prototypes for type checking purposes. That's why this type has the
+ * `$CombinedState` symbol property. Without the property, this type would
+ * match any object. The symbol doesn't really exist because it's an internal
+ * (i.e. not exported), and internally we never check its value. Since it's a
+ * symbol property, it's not expected to be unumerable, and the value is
+ * typed as always undefined, so its never expected to have a meaningful
+ * value anyway. It just makes this type distinquishable from plain `{}`.
+ */
+export type CombinedState<S> = { readonly [$CombinedState]?: undefined } & S
+
+/**
+ * Recursively makes combined state objects partial. Only combined state _root
+ * objects_ (i.e. the generated higher level object with keys mapping to
+ * individual reducers) are partial.
+ */
+export type PreloadedState<S> = Required<S> extends {
+  [$CombinedState]: undefined
+}
+  ? S extends CombinedState<infer S1>
+    ? {
+        [K in keyof S1]?: S1[K] extends object ? PreloadedState<S1[K]> : S1[K]
+      }
+    : never
+  : {
+      [K in keyof S]: S[K] extends object ? PreloadedState<S[K]> : S[K]
+    }
+
 /* reducers */
 
 /**
@@ -136,13 +176,16 @@ export type ActionFromReducersMapObject<M> = M extends ReducersMapObject<
  */
 export function combineReducers<S>(
   reducers: ReducersMapObject<S, any>
-): Reducer<S>
+): Reducer<CombinedState<S>>
 export function combineReducers<S, A extends Action = AnyAction>(
   reducers: ReducersMapObject<S, A>
-): Reducer<S, A>
+): Reducer<CombinedState<S>, A>
 export function combineReducers<M extends ReducersMapObject<any, any>>(
   reducers: M
-): Reducer<StateFromReducersMapObject<M>, ActionFromReducersMapObject<M>>
+): Reducer<
+  CombinedState<StateFromReducersMapObject<M>>,
+  ActionFromReducersMapObject<M>
+>
 
 /* store */
 
@@ -316,7 +359,7 @@ export interface StoreCreator {
   ): Store<S & StateExt, A> & Ext
   <S, A extends Action, Ext, StateExt>(
     reducer: Reducer<S, A>,
-    preloadedState?: DeepPartial<S>,
+    preloadedState?: PreloadedState<S>,
     enhancer?: StoreEnhancer<Ext>
   ): Store<S & StateExt, A> & Ext
 }
@@ -380,7 +423,7 @@ export type StoreEnhancerStoreCreator<Ext = {}, StateExt = {}> = <
   A extends Action = AnyAction
 >(
   reducer: Reducer<S, A>,
-  preloadedState?: DeepPartial<S>
+  preloadedState?: PreloadedState<S>
 ) => Store<S & StateExt, A> & Ext
 
 /* middleware */

test/typescript/enhancers.ts

@@ -1,11 +1,5 @@
-import {
-  StoreEnhancer,
-  Action,
-  AnyAction,
-  Reducer,
-  createStore,
-  DeepPartial
-} from 'redux'
+import { PreloadedState } from '../../index'
+import { StoreEnhancer, Action, AnyAction, Reducer, createStore } from 'redux'
 
 interface State {
   someField: 'string'
@@ -43,10 +37,10 @@ function stateExtension() {
     A extends Action = AnyAction
   >(
     reducer: Reducer<S, A>,
-    preloadedState?: DeepPartial<S>
+    preloadedState?: PreloadedState<S>
   ) => {
     const wrappedReducer: Reducer<S & ExtraState, A> = null as any
-    const wrappedPreloadedState: S & ExtraState = null as any
+    const wrappedPreloadedState: PreloadedState<S & ExtraState> = null as any
     return createStore(wrappedReducer, wrappedPreloadedState)
   }
 

test/typescript/store.ts

@@ -57,21 +57,45 @@ const funcWithStore = (store: Store<State, DerivedAction>) => {}
 const store: Store<State> = createStore(reducer)
 
 const storeWithPreloadedState: Store<State> = createStore(reducer, {
+  a: 'a',
+  b: { c: 'c', d: 'd' }
+})
+// typings:expect-error
+const storeWithBadPreloadedState: Store<State> = createStore(reducer, {
   b: { c: 'c' }
 })
 
 const storeWithActionReducer = createStore(reducerWithAction)
 const storeWithActionReducerAndPreloadedState = createStore(reducerWithAction, {
-  b: { c: 'c' }
+  a: 'a',
+  b: { c: 'c', d: 'd' }
 })
 funcWithStore(storeWithActionReducer)
 funcWithStore(storeWithActionReducerAndPreloadedState)
 
+// typings:expect-error
+const storeWithActionReducerAndBadPreloadedState = createStore(
+  reducerWithAction,
+  {
+    b: { c: 'c' }
+  }
+)
+
 const enhancer: StoreEnhancer = next => next
 
 const storeWithSpecificEnhancer: Store<State> = createStore(reducer, enhancer)
 
 const storeWithPreloadedStateAndEnhancer: Store<State> = createStore(
+  reducer,
+  {
+    a: 'a',
+    b: { c: 'c', d: 'd' }
+  },
+  enhancer
+)
+
+// typings:expect-error
+const storeWithBadPreloadedStateAndEnhancer: Store<State> = createStore(
   reducer,
   {
     b: { c: 'c' }