add unwrapResult helper

Author: phryneas

etc/redux-toolkit.api.md

@@ -121,6 +121,13 @@ export function createAsyncThunk<ActionType extends string, Returned, ActionPara
         args: ActionParams;
         requestId: string;
     }>;
+    unwrapResult: (returned: import("./createAction").PayloadAction<Returned, string, {
+        args: ActionParams;
+        requestId: string;
+    }, never> | import("./createAction").PayloadAction<undefined, string, {
+        args: ActionParams;
+        requestId: string;
+    }, Error>) => Returned;
 };
 
 // @alpha (undocumented)

src/createAsyncThunk.ts

@@ -2,6 +2,8 @@ import { Dispatch } from 'redux'
 import nanoid from 'nanoid'
 import { createAction } from './createAction'
 
+type Await<P> = P extends PromiseLike<infer T> ? T : P
+
 type AsyncThunksArgs<S, E, D extends Dispatch = Dispatch> = {
   dispatch: D
   getState: S
@@ -129,5 +131,19 @@ export function createAsyncThunk<
     }
   }
 
-  return Object.assign(actionCreator, { pending, rejected, fulfilled })
+  function unwrapResult(
+    returned: Await<ReturnType<ReturnType<typeof actionCreator>>>
+  ) {
+    if (rejected.match(returned)) {
+      throw returned.error
+    }
+    return returned.payload
+  }
+
+  return Object.assign(actionCreator, {
+    pending,
+    rejected,
+    fulfilled,
+    unwrapResult
+  })
 }

type-tests/files/createAsyncThunk.typetest.ts

@@ -1,5 +1,6 @@
 import { createAsyncThunk, Dispatch, createReducer, AnyAction } from 'src'
 import { ThunkDispatch } from 'redux-thunk'
+import { promises } from 'fs'
 
 function expectType<T>(t: T) {
   return t
@@ -29,7 +30,8 @@ function fn() {}
       })
   )
 
-  const result = await dispatch(async(3))
+  const promise = dispatch(async(3))
+  const result = await promise
 
   if (async.fulfilled.match(result)) {
     expectType<ReturnType<typeof async['fulfilled']>>(result)
@@ -40,4 +42,15 @@ function fn() {}
     // typings:expect-error
     expectType<ReturnType<typeof async['fulfilled']>>(result)
   }
+
+  promise
+    .then(async.unwrapResult)
+    .then(result => {
+      expectType<number>(result)
+      // typings:expect-error
+      expectType<Error>(result)
+    })
+    .catch(error => {
+      // catch is always any-typed, nothing we can do here
+    })
 })()