strong typed redux action with typescript@2.8

strong-typed-redux-action.ts

function createAction<A extends ActionTypes>(type: A): TypedAction<A>
function createAction<A extends ActionTypes, P>(type: A, payload: P): TypePayloadAction<A, P>
function createAction(type, payload?) {
  return payload !== undefined ? {type, payload} : {type}
}

enum ActionTypes {
  ACTION_1 = 'action 1',
  ACTION_2 = 'action 2'
}

const actions = {
  actionA: () => createAction(ActionTypes.ACTION_1, {a:1}),
  actionB: () => createAction(ActionTypes.ACTION_2, {b:1}),
}

interface TypedAction<A> {
  type: A
}
interface TypePayloadAction<A, P> extends TypedAction<A> {
  payload: P
}

function reducer(state: State, action: ReturnType<typeof actions[keyof typeof actions]>): State {
  switch (action.type) {
    case ActionTypes.ACTION_1:
      action.payload.a
      // action.payload.b // error
      return {}
    case ActionTypes.ACTION_2:
      // action.payload.a // error
      action.payload.b
      return {}
  }
}

interface State {

}