Inferred generic type of reducers Number/String/Boolean instead of number/string/boolean

[lephyrus]

When the state is a number, string or boolean, and the reducer function uses no parameters (() => newState), the generic type of the action reducer is wrongly inferred to be Number, String or Boolean.

This leads to Typescript errors either in createReducer() itself (depending on the order or the action reducers) or later on.

This was not an issue in @ngrx/store v10 - I hit this as I've upgraded to v11.

Minimal reproduction of the bug/regression with instructions:

Reproduction in the Typescript playground.

import { Action, ActionReducer, createAction, createReducer, on, props } from '@ngrx/store';

const initialState: number = 0;

const setAction = createAction('set', props<{ value: number }>());
const resetAction = createAction('reset');

export const failingReducer = createReducer(
  initialState,
  on(setAction, (_state, { value }) => value),  // Typescript error
  on(resetAction, () => initialState),
);

export const workingReducer = createReducer(
  initialState,
  on(resetAction, () => initialState),
  on(setAction, (_state, { value }) => value),
);

export const expectedActionReducer: ActionReducer<number, Action> = workingReducer;  // Typescript error
export const actualActionReducer: ActionReducer<Number, Action> = workingReducer;

Expected behavior:

If the state is number, string, or boolean, reducers' inferred generic type should be the same, even if there is a "reset-style" action reducer.

Versions of NgRx, Angular, Node, affected browser(s) and operating system(s):

NgRx: 11.0.1
Typescript: 4.1.5 / 4.2.3

Other information:

The workaround is to add the state argument to the reducer function even though it is not needed:

export const reducer = createReducer(
  initialState,
  on(setAction, (_state, { value }) => value),
  on(resetAction, (_state) => initialState),
);

// works!
export const expectedActionReducer: ActionReducer<number, Action> = reducer;

I would be willing to submit a PR to fix this issue

[ ] Yes (Assistance is provided if you need help submitting a pull request)
[X] No

[parloti]

Now I can no longer use on() with a generic state. Apparently, problem with type inference.

Play