create a typesafe reducer with a map of immer action functions

createReducer.ts

import { Draft, produce } from "immer"

type ValueOf<T> = T[keyof T]

type ActionType<
  ActionMap extends Record<string, (state: any, arg: any) => void>
> = ValueOf<
  {
    [K in keyof ActionMap]: ActionMap[K] extends (
      state: any,
      arg: infer Payload
    ) => void
      ? unknown extends Payload
        ? { type: K }
        : { type: K; payload: Payload }
      : never
  }
>

function createReducer<State>() {
  return function configureActions<
    ActionMap extends Record<string, (state: Draft<State>, arg: any) => void>
  >(actionMap: ActionMap) {
    return function reducer(state: State, action: ActionType<ActionMap>) {
      const handler = actionMap[action.type]
      return handler ? produce(state, (draft) => handler(draft, action)) : state
    }
  }
}

const reducer = createReducer<{ readonly count: number }>()({
  increment(state, amount: number) {
    state.count += 1
  },
  decrement(state) {
    state.count -= 1
  },
  reset(state) {
    state.count = 0
  },
})

reducer({ count: 0 }, { type: "increment", payload: 1 })
reducer({ count: 0 }, { type: "reset" })