Allow returning arbitrary data from the handleAction function

src/handle-action.test.ts

@@ -32,3 +32,35 @@ test('handles specific action with payload', () => {
   expect(newState2).toEqual({ counter: 0 });
   expect(newState2).toBe(state);
 });
+
+test('handles specific action with payload by returning value directly', () => {
+  const ac1 = createAction<{ num: number }>('foo3');
+  const state: { readonly counter: number } = { counter: 0 };
+  const re = handleAction<typeof state>(ac1, (draft, { payload }) => ({
+    counter: draft.counter + payload.num,
+  }), state);
+  const newState1 = re(state, ac1({ num: 7 }));
+  expect(newState1).toEqual({ counter: 7 });
+  expect(newState1).not.toBe(state);
+});
+
+test('handles specific action with payload and ignores directly returned value if draft is mutated', () => {
+  const ac1 = createAction<{ num: number }>('foo4');
+  const state: { readonly counter: number } = { counter: 0 };
+  const re = handleAction<typeof state>(ac1, (draft, { payload }) => {
+    draft.counter += payload.num;
+    return 'unintended return value';
+  }, state);
+  const newState1 = re(state, ac1({ num: 10 }));
+  expect(newState1).toEqual({ counter: 10 });
+  expect(newState1).not.toBe(state);
+});
+
+test('handles specific action and uses previous state if directly return value is undefined', () => {
+  const ac1 = createAction<void>('foo5');
+  const state: { readonly baz: number } = { baz: 0 };
+  const re = handleAction<typeof state>(ac1, () => undefined, state);
+  const newState1 = re(state, ac1());
+  expect(newState1).toEqual({ baz: 0 });
+  expect(newState1).toBe(state);
+});

src/handle-action.ts

@@ -23,8 +23,14 @@ export default function handleAction<S, AC extends TsActionCreator<any> = any>(
   return (state: S | undefined, action: ReturnType<AC>) => {
     if (action.type === ac.type && state) {
       const draft = createDraft(state);
-      re(draft, action);
-      return finishDraft(draft);
+      const reResult = re(draft, action);
+      const finishedDraft = finishDraft(draft);
+
+      if (finishedDraft === state && reResult !== undefined) {
+        return reResult;
+      }
+
+      return finishedDraft;
     }
     return (state || s) as any;
   };