Previous implementations of Redux reducers using combineReducers broken in 2.4.1

[jimsugg]

TypeScript Version: 2.4.1
Code

// From Redux declaration file
interface Action {
  type: any;
}
type Reducer<S> = <A extends Action>(state: S, action: A) => S;
interface ReducersMapObject {
  [key: string]: Reducer<any>;
}
declare function combineReducers<S>(reducers: ReducersMapObject): Reducer<S>;

// User code
interface BsDmBaseAction extends Action {
  type: string;  // override to a string type
  payload: number;
}

interface State1 {
  id: string;
  data: number;
}

function statesById(state: State1[] = [], action: BsDmBaseAction): State1[] {
  switch (action.type) {
    case 'STOP': return [{id: 'status', data: action.payload}];
  }
  return state;
}

function allIds(state: string[] = [], action: BsDmBaseAction): string[] {
  switch (action.type) {
    case 'STOP': return ['status'];
  }
  return state;
}

const bigReducer = combineReducers({
  statesById,
  allIds,
});

Expected behavior:
No errors. This has been working since Typescript 1.8, and works in 2.3.4.

Actual behavior:
Starting with Typescript 2.4.1, the following error is emitted for the combineReducers call:
Error:(40, 36) TS2345:Argument of type '{ statesById: (state: State1[], action: BsDmBaseAction) => State1[]; allIds: (state: string[], ac...' is not assignable to parameter of type 'ReducersMapObject'.
Property 'statesById' is incompatible with index signature.
Type '(state: State1[], action: BsDmBaseAction) => State1[]' is not assignable to type 'Reducer'.
Types of parameters 'action' and 'action' are incompatible.
Type 'A' is not assignable to type 'BsDmBaseAction'.
Type 'Action' is not assignable to type 'BsDmBaseAction'.
Property 'payload' is missing in type 'Action'.

Notes
I do not see any explanation in the various notes on 'breaking changes' to explain this. There are no weak types here, which was involved in the main 'breaking change' description.

[weswigham]

This would probably be a result of strict contravariance for callback parameters.

[jimsugg]

It may be indeed, although I must admit I still fail to see how it is a 'bug' as described in the paragraph you cite. In any case, it is a pretty significant disaster for those of us using Redux, assuming we want to keep using Typescript also. Thanks for pointing to the paragraph though.

[HerringtonDarkholme]

@weswigham I think this is a valid TypeScript bug. If I change the reducer to type Reducer<S> = (state: S, action: Action) => S;, it compiles.

[jimsugg]

Fair enough, it does. Let me see if I can extend that approach into our more complex real world scenario. Also, now I'm curious if it has always been the case that your syntax would work. Why was the other more obtuse syntax ('') ever used? Anyway, thanks.

[jimsugg]

Well, that works, but the change has to be in the Redux index.d.ts.

[weswigham]

There's already an open PR.

[jimsugg]

Yep - found that - I have pulled that down and am testing it. It works well. There's a suggestion for an alternative that I will test today. A big issue is that it's going to be really clunky until they actually get the fix into the Redux distribution. I can get that into my work ok, but rolling it out to my team will be problematic until it's actually part of Redux.

[DonikaV]

@jimsugg
Hey, i tried to use your suggestion but have error in console
Uncaught ReferenceError: combineReducers is not defined

My reducers/index.ts looks like

import { sessionReducer } from './session';
import { routerReducer } from 'react-router-redux';
interface Action {
	type: any;
}
type Reducer<S> = (state: S, action: Action) => S;

interface ReducersMapObject {
	[key: string]: Reducer<any>;
}
declare function combineReducers<S>(reducers: ReducersMapObject): Reducer<S>;

export const reducers =  combineReducers({
	sessionReducer,
        routing: routerReducer,
});

Can you help me? Thanks.

[willisplummer]

i think this issue can be closed! reduxjs/redux#2467 fixed for me

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.

[lenguyenthanh]

It is broken again with Typescript 2.6.1 and Redux 3.7.2
Here is my code:

import State from './state'
import { defaultState } from './state'
import { combineReducers, Reducer } from 'redux'
import { Dispatch, Store, Action } from 'redux'
import { reducer as count } from '../feature/counter/counterScreen'

export enum keys {
  INCREMENT = 'increment',
  DECREMENT = 'decrement',
}

interface IncrementAction {
  readonly type: keys.INCREMENT
  readonly payload: {
    readonly size: number
  }
}
const incrementAction = (size: number): IncrementAction => ({
  type: keys.INCREMENT,
  payload: {
    size: size,
  },
})

interface DecrementAction {
  readonly type: keys.DECREMENT
  readonly payload: {
    readonly size: number
  }
}
const decrementAction = (size: number): DecrementAction => ({
  type: keys.DECREMENT,
  payload: {
    size: size,
  },
})

export type ActionTypes = IncrementAction | DecrementAction

/** REDUCER */
export function reducer(state = defaultState.count, action: ActionTypes) {
  switch (action.type) {
    case keys.DECREMENT:
      return state - action.payload.size
    case keys.INCREMENT:
      return state + action.payload.size
    default:
      return state
  }
}

const rootReducers: Reducer<State> = combineReducers({ count })

export default rootReducers

and here is error:

yarn build v0.27.5
$ yarn run clean && yarn run lint && yarn tsc --
src/app/reducers.ts(53,54): error TS2345: Argument of type '{ count: (state: number | undefined, action: ActionTypes) => number; }' is not assignable to parameter of type 'ReducersMapObject'.
  Property 'count' is incompatible with index signature.
    Type '(state: number | undefined, action: ActionTypes) => number' is not assignable to type 'Reducer<any>'.
      Types of parameters 'action' and 'action' are incompatible.
        Type 'AnyAction' is not assignable to type 'ActionTypes'.
          Type 'AnyAction' is not assignable to type 'DecrementAction'.
            Property 'payload' is missing in type 'AnyAction'.

It works well if I change Reducer definition in index.d.ts file:

export type Reducer<S, A extends AnyAction> = (state: S, action: A) => S;

And my reducer:

const rootReducers: Reducer<State, ActionTypes> = combineReducers({ count })

[jhalborg]

Confirmed, TS 2.6.1 and redux 3.7.2 with similar setup as @lenguyenthanh mentions

[lenguyenthanh]

@jhalborg I created an issue in Redux repository: reduxjs/redux#2709.
So I'm using Redux 3.7.2 with index.d.ts file from Redux@4.0.0-beta.1. It works for me.