alternative approach for TS<3.5

Author: phryneas

etc/redux-toolkit.api.md

@@ -202,13 +202,9 @@ export { ThunkAction }
 // @public
 export type ValidateSliceCaseReducers<S, ACR extends SliceCaseReducers<S>> = ACR & {
     [T in keyof ACR]: ACR[T] extends {
-        prepare(...a: never[]): {
-            payload: infer P;
-            meta?: infer M;
-            error?: infer E;
-        };
+        reducer(s: S, action?: infer A): any;
     } ? {
-        reducer(s: S, action: PayloadAction<P, string, M, E>): any;
+        prepare(...a: never[]): Omit<A, 'type'>;
     } : {};
 };
 

src/createSlice.ts

@@ -12,6 +12,7 @@ import {
   ActionReducerMapBuilder,
   executeReducerBuilderCallback
 } from './mapBuilders'
+import { Omit } from './tsHelpers'
 
 /**
  * An action creator atttached to a slice.
@@ -188,12 +189,10 @@ export type ValidateSliceCaseReducers<
 > = ACR &
   {
     [T in keyof ACR]: ACR[T] extends {
-      prepare(
-        ...a: never[]
-      ): { payload: infer P; meta?: infer M; error?: infer E }
+      reducer(s: S, action?: infer A): any
     }
       ? {
-          reducer(s: S, action: PayloadAction<P, string, M, E>): any
+          prepare(...a: never[]): Omit<A, 'type'>
         }
       : {}
   }

src/tsHelpers.ts

@@ -87,3 +87,5 @@ type UnionToIntersection<U> = (U extends any
   : never) extends ((k: infer I) => void)
   ? I
   : never
+
+export type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>

type-tests/files/createSlice.typetest.ts

@@ -70,7 +70,7 @@ function expectType<T>(t: T) {
           ? payload.reduce((acc, val) => acc * val, state)
           : state * payload,
       addTwo: {
-        reducer: (s, { payload }) => s + payload,
+        reducer: (s, { payload }: PayloadAction<number>) => s + payload,
         prepare: (a: number, b: number) => ({
           payload: a + b
         })
@@ -212,8 +212,8 @@ function expectType<T>(t: T) {
       },
       // case: meta is typed differently in the reducer than returned from prepare
       testErroneousMeta: {
-        // typings:expect-error
         reducer(_, action: PayloadAction<number, string, 'meta', 'error'>) {},
+        // typings:expect-error
         prepare: (payload: number) => ({
           payload,
           meta: 1,
@@ -222,8 +222,8 @@ function expectType<T>(t: T) {
       },
       // case: error is typed differently in the reducer than returned from prepare
       testErroneousError: {
-        // typings:expect-error
         reducer(_, action: PayloadAction<number, string, 'meta', 'error'>) {},
+        // typings:expect-error
         prepare: (payload: number) => ({
           payload,
           meta: 'meta' as 'meta',