Type-safe Redux

type-safe-redux.ts

interface BasicAction<T> {
  type: T
}

interface PayloadAction<T, P> extends BasicAction<T> {
  payload: P
}

interface ErrorAction<T> extends BasicAction<T> {
  error: Error
}

type PayloadErrorAction<T, P> = PayloadAction<T, P> & ErrorAction<T>

type ActionA = BasicAction<'ACTION_A'>
type ActionB = PayloadAction<'ACTION_B', {foo: string}>
type ActionC = PayloadErrorAction<'ACTION_C', {bar: number}>

type Action = ActionA | ActionB | ActionC

const reducer = (state: any, action: Action) => {
  switch (action.type) {
    case 'ACTION_A':
      // console.log(action.payload) // FAIL!
      // console.log(action.error) // FAIL!
      break

    case 'ACTION_B':
      console.log(action.payload.foo)
      // console.log(action.error) // FAIL!
      break

    case 'ACTION_C':
      console.log(action.payload.bar)
      console.log(action.error)
      break

    default:
      break
  }
}