fix(store): fix typing of on fn

Author: aokrushk

modules/store/spec/types/reducer_creator.spec.ts

@@ -90,5 +90,39 @@ describe('createReducer()', () => {
         on(foo, (state, action) => { const bar: string = action.bar; return state; });
       `).toFail(/'bar' does not exist on type/);
     });
+
+    it('should infer the typed based on state and actions type with action used in on function', () => {
+      expectSnippet(`
+        interface State { name: string };
+        const foo = createAction('FOO', props<{ foo: string }>());
+        const onFn = on(foo, (state: State, action) => ({  name: action.foo }));
+      `).toInfer(
+        'onFn',
+        `
+      ReducerTypes<{
+        name: string;
+    }, [ActionCreator<"FOO", (props: {
+        foo: string;
+    }) => {
+        foo: string;
+    } & TypedAction<"FOO">>]>
+    `
+      );
+    });
+
+    it('should infer the typed based on state and actions type without action', () => {
+      expectSnippet(`
+        interface State { name: string };
+        const foo = createAction('FOO');
+        const onFn = on(foo, (state: State) => ({ name: 'some value' }));
+      `).toInfer(
+        'onFn',
+        `
+      ReducerTypes<{
+        name: string;
+    }, [ActionCreator<"FOO", () => TypedAction<"FOO">>]>
+    `
+      );
+    });
   });
 });

modules/store/src/reducer_creator.ts

@@ -40,15 +40,12 @@ export interface OnReducer<State, Creators extends readonly ActionCreator[]> {
  * ```
  */
 export function on<State, Creators extends readonly ActionCreator[]>(
-  ...args: [
-    ...creators: Creators,
-    reducer: OnReducer<State extends infer S ? S : never, Creators>
-  ]
+  ...args: [...creators: Creators, reducer: OnReducer<State, Creators>]
 ): ReducerTypes<State, Creators> {
-  // This could be refactored when TS releases the version with this fix:
-  // https://github.com/microsoft/TypeScript/pull/41544
-  const reducer = args.pop() as OnReducer<any, Creators>;
-  const types = (args as unknown as Creators).map(
+  const creators = args.slice(0, -1) as unknown as Creators;
+  const reducer = args[args.length - 1] as OnReducer<State, Creators>;
+
+  const types = creators.map(
     (creator) => creator.type
   ) as unknown as ExtractActionTypes<Creators>;
   return { reducer, types };