actionCreators.ts

import * as actionTypes from "./actionTypes";

export function dispatchFirstAction(payload: string) {
  return {
    type: actionTypes.FirstAction,
    payload
  };
}

export function dispatchSecondAction(payload: number) {
  return {
    type: actionTypes.SecondAction,
    payload
  };
}

actions.ts

import * as actionCreators from "./actionCreators";
import * as actionTypes from "./actionTypes";

export type InferValueTypes<T> = T extends { [key: string]: infer U }
  ? U
  : never;

type State = { value: string };
export default function reducer(
  state: State = { value: "hello" },
  action: ReturnType<InferValueTypes<typeof actionCreators>>
): State {
  switch (action.type) {
    case actionTypes.FirstAction:
      return { value: action.payload };
    case actionTypes.SecondAction:
      return { value: state.value + action.payload };
    default:
      return state;
  }
}

actionTypes.ts

export const FirstAction = "TestReducer/FirstAction" as const;
export const SecondAction = "TestReducer/SecondAction" as const;