Property 'payload' is missing in type 'AnyAction'

[huan]

Property 'payload' is missing in type 'AnyAction'

[huan]

import { Reducer } from 'redux'

const count: Reducer<number> = (state: number) => {
  return state
}

void count

The above code will yield the following error message:

error TS2322: Type '(state: number) => number' is not assignable to type 'Reducer<number, AnyAction>'.
  Types of parameters 'state' and 'state' are incompatible.
    Type 'number | undefined' is not assignable to type 'number'.
      Type 'undefined' is not assignable to type 'number'.

3 const count: Reducer<number> = (state: number) => {

It turns out that we are declaring a function that can accept the state as the undefined | number type, however, the implemented function can only accept the number type of the argument, which can not satisfy the declaration.