feat(action): support optional error field

docs/api/createAction.md

@@ -79,7 +79,7 @@ console.log(addTodo('Write more docs'))
  **/
 ```
 
-If provided, all arguments from the action creator will be passed to the prepare callback, and it should return an object with the `payload` field (otherwise the payload of created actions will be `undefined`). Additionally, the object can have a `meta` field that will also be added to created actions. This may contain extra information about the action. These two fields (`payload` and `meta`) adhere to the specification of [Flux Standard Actions](https://github.com/redux-utilities/flux-standard-action#actions).
+If provided, all arguments from the action creator will be passed to the prepare callback, and it should return an object with the `payload` field (otherwise the payload of created actions will be `undefined`). Additionally, the object can have a `meta` and/or an `error` field that will also be added to created actions. `meta` may contain extra information about the action, `error` may contain details about the action failure. These three fields (`payload`, `meta` and `error`) adhere to the specification of [Flux Standard Actions](https://github.com/redux-utilities/flux-standard-action#actions).
 
 **Note:** The type field will be added automatically.
 

src/createAction.test.ts

@@ -48,6 +48,50 @@ describe('createAction', () => {
     })
   })
 
+  describe('when passing a prepareAction method returning a payload and error', () => {
+    it('should use the payload returned from the prepareAction method', () => {
+      const actionCreator = createAction('A_TYPE', (a: number) => ({
+        payload: a * 2,
+        error: true
+      }))
+      expect(actionCreator(5).payload).toBe(10)
+    })
+    it('should use the error returned from the prepareAction method', () => {
+      const actionCreator = createAction('A_TYPE', (a: number) => ({
+        payload: a * 2,
+        error: true
+      }))
+      expect(actionCreator(10).error).toBe(true)
+    })
+  })
+
+  describe('when passing a prepareAction method returning a payload, meta and error', () => {
+    it('should use the payload returned from the prepareAction method', () => {
+      const actionCreator = createAction('A_TYPE', (a: number) => ({
+        payload: a * 2,
+        meta: a / 2,
+        error: true
+      }))
+      expect(actionCreator(5).payload).toBe(10)
+    })
+    it('should use the error returned from the prepareAction method', () => {
+      const actionCreator = createAction('A_TYPE', (a: number) => ({
+        payload: a * 2,
+        meta: a / 2,
+        error: true
+      }))
+      expect(actionCreator(10).error).toBe(true)
+    })
+    it('should use the meta returned from the prepareAction method', () => {
+      const actionCreator = createAction('A_TYPE', (a: number) => ({
+        payload: a * 2,
+        meta: a / 2,
+        error: true
+      }))
+      expect(actionCreator(10).meta).toBe(5)
+    })
+  })
+
   describe('when passing a prepareAction that accepts multiple arguments', () => {
     it('should pass all arguments of the resulting actionCreator to prepareAction', () => {
       const actionCreator = createAction(

src/createAction.ts

@@ -8,24 +8,29 @@ import { IsUnknownOrNonInferrable } from './tsHelpers'
  * @template P The type of the action's payload.
  * @template T the type used for the action type.
  * @template M The type of the action's meta (optional)
+ * @template E The type of the action's error (optional)
  */
 export type PayloadAction<
   P = void,
   T extends string = string,
-  M = void
-> = WithOptionalMeta<M, WithPayload<P, Action<T>>>
+  M = void,
+  E = void
+> = WithOptional<M, E, WithPayload<P, Action<T>>>
 
 export type PrepareAction<P> =
   | ((...args: any[]) => { payload: P })
   | ((...args: any[]) => { payload: P; meta: any })
+  | ((...args: any[]) => { payload: P; meta: any; error: any })
 
 export type ActionCreatorWithPreparedPayload<
   PA extends PrepareAction<any> | void,
   T extends string = string
 > = WithTypeProperty<
   T,
   PA extends PrepareAction<infer P>
-    ? (...args: Parameters<PA>) => PayloadAction<P, T, MetaOrVoid<PA>>
+    ? (
+        ...args: Parameters<PA>
+      ) => PayloadAction<P, T, MetaOrVoid<PA>, ErrorOrVoid<PA>>
     : void
 >
 
@@ -113,9 +118,13 @@ export function createAction(type: string, prepareAction?: Function) {
       if (!prepared) {
         throw new Error('prepareAction did not return an object')
       }
-      return 'meta' in prepared
-        ? { type, payload: prepared.payload, meta: prepared.meta }
-        : { type, payload: prepared.payload }
+
+      return {
+        type,
+        payload: prepared.payload,
+        ...('meta' in prepared && { meta: prepared.meta }),
+        ...('error' in prepared && { error: prepared.error })
+      }
     }
     return { type, payload: args[0] }
   }
@@ -147,7 +156,9 @@ type Diff<T, U> = T extends U ? never : T
 
 type WithPayload<P, T> = T & { payload: P }
 
-type WithOptionalMeta<M, T> = T & ([M] extends [void] ? {} : { meta: M })
+type WithOptional<M, E, T> = T &
+  ([M] extends [void] ? {} : { meta: M }) &
+  ([E] extends [void] ? {} : { error: E })
 
 type WithTypeProperty<T, MergeIn> = {
   type: T
@@ -165,6 +176,12 @@ type MetaOrVoid<PA extends PrepareAction<any>> = ReturnType<PA> extends {
   ? M
   : void
 
+type ErrorOrVoid<PA extends PrepareAction<any>> = ReturnType<PA> extends {
+  error: infer E
+}
+  ? E
+  : void
+
 type IfMaybeUndefined<P, True, False> = [undefined] extends [P] ? True : False
 
 type IfVoid<P, True, False> = [void] extends [P] ? True : False

type-tests/files/createAction.typetest.ts

@@ -156,6 +156,8 @@ function expectType<T>(p: T): T {
 
   // typings:expect-error
   expectType<string>(strLenAction('test').payload)
+  // typings:expect-error
+  expectType<boolean>(strLenAction('test').error)
 }
 
 /*
@@ -171,6 +173,38 @@ function expectType<T>(p: T): T {
 
   // typings:expect-error
   expectType<string>(strLenMetaAction('test').meta)
+  // typings:expect-error
+  expectType<boolean>(strLenMetaAction('test').error)
+}
+
+/*
+ * Test: adding boolean error with prepareAction
+ */
+{
+  const boolErrorAction = createAction('boolError', (payload: string) => ({
+    payload,
+    error: true
+  }))
+
+  expectType<boolean>(boolErrorAction('test').error)
+
+  // typings:expect-error
+  expectType<string>(boolErrorAction('test').error)
+}
+
+/*
+ * Test: adding string error with prepareAction
+ */
+{
+  const strErrorAction = createAction('strError', (payload: string) => ({
+    payload,
+    error: 'this is an error'
+  }))
+
+  expectType<string>(strErrorAction('test').error)
+
+  // typings:expect-error
+  expectType<boolean>(strErrorAction('test').error)
 }
 
 /*