add cache to combineSlices

Author: EskiMojo14

packages/toolkit/src/combineSlices.ts

@@ -323,6 +323,7 @@ const stateProxyMap = new WeakMap<object, object>()
 const createStateProxy = <State extends object>(
   state: State,
   reducerMap: Partial<Record<string, Reducer>>,
+  initialStateCache: Record<string, unknown>,
 ) =>
   getOrInsertComputed(
     stateProxyMap,
@@ -332,20 +333,24 @@ const createStateProxy = <State extends object>(
         get: (target, prop, receiver) => {
           if (prop === ORIGINAL_STATE) return target
           const result = Reflect.get(target, prop, receiver)
+          const propString = prop.toString()
           if (typeof result === 'undefined') {
-            const reducer = reducerMap[prop.toString()]
+            const cached = initialStateCache[propString]
+            if (typeof cached !== 'undefined') return cached
+            const reducer = reducerMap[propString]
             if (reducer) {
               // ensure action type is random, to prevent reducer treating it differently
               const reducerResult = reducer(undefined, { type: nanoid() })
               if (typeof reducerResult === 'undefined') {
                 throw new Error(
-                  `The slice reducer for key "${prop.toString()}" returned undefined when called for selector(). ` +
+                  `The slice reducer for key "${propString}" returned undefined when called for selector(). ` +
                     `If the state passed to the reducer is undefined, you must ` +
                     `explicitly return the initial state. The initial state may ` +
                     `not be undefined. If you don't want to set a value for this reducer, ` +
                     `you can use null instead of undefined.`,
                 )
               }
+              initialStateCache[propString] = reducerResult
               return reducerResult
             }
           }
@@ -382,6 +387,8 @@ export function combineSlices<Slices extends Array<AnySliceLike | ReducerMap>>(
 
   combinedReducer.withLazyLoadedSlices = () => combinedReducer
 
+  const initialStateCache: Record<string, unknown> = {}
+
   const inject = (
     slice: AnySliceLike,
     config: InjectConfig = {},
@@ -406,6 +413,10 @@ export function combineSlices<Slices extends Array<AnySliceLike | ReducerMap>>(
       return combinedReducer
     }
 
+    if (config.overrideExisting && currentReducer !== reducerToInject) {
+      delete initialStateCache[reducerPath]
+    }
+
     reducerMap[reducerPath] = reducerToInject
 
     reducer = getReducer()
@@ -423,6 +434,7 @@ export function combineSlices<Slices extends Array<AnySliceLike | ReducerMap>>(
           createStateProxy(
             selectState ? selectState(state as any, ...args) : state,
             reducerMap,
+            initialStateCache,
           ),
           ...args,
         )