Redux Reducer type error

[zhahaoyu]

I am trying to define reducers used in redux. I don't know why this definition not works.

TypeScript Version: 2.4.1

Code

// Type definitions from 'redux'
export interface Action {
  type: any;
}

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

// My code
import { Action, Reducer } from 'redux';

enum Types {
  add,
}

interface Increment extends Action {
  type: Types.add;
  value: number;
}

const count: Reducer<number> = (state: number, action: Increment) => {
  if (action.type === Types.add) {
    return state + action.value;
  }
  return state;
};

Expected behavior:
This should work!

Actual behavior:

(14,7): error TS2322: Type '(state: number, action: Increment) => number' is not assignable to type 'Reducer<number>'.
  Types of parameters 'action' and 'action' are incompatible.
    Type 'A' is not assignable to type 'Increment'.
      Type 'Action' is not assignable to type 'Increment'.
        Property 'value' is missing in type 'Action'.

[zhahaoyu]

This works in Typescript 2.3.4

[kitsonk]

Dupe of #16985

[DanielRosenwasser]

Not exactly a dupe of that. This is due to stricter generic checks though, which that issue is related to.

You're trying to assign a (state: number, action: Increment) => number to a <A extends Action>(state: number, action: A) => number, meaning that you're expecting more than you may potentially be given. So your Reducer definition is incorrect.

Check out reduxjs/redux#2467 for the fix I put out to Redux which includes an explanation.

[mhegazy]

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