Function typeGuards

test/typescript/reducers.ts:180–232

* Reducer definition using type guards.

()

 * Reducer definition using type guards.
 */
function typeGuards() {
  function isAction<A extends Action>(action: Action, type: any): action is A {
    return action.type === type
  }

  type State = number

  interface IncrementAction {
    type: 'INCREMENT'
    count?: number
  }

  interface DecrementAction {
    type: 'DECREMENT'
    count?: number
  }

  const reducer: Reducer<State, AnyAction> = (state = 0, action) => {
    if (isAction<IncrementAction>(action, 'INCREMENT')) {
      // TODO: this doesn't seem to work correctly with UnknownAction - `action` becomes `UnknownAction & IncrementAction`
      // Action shape is determined by the type guard returned from `isAction`
      // @ts-expect-error
      action.wrongField

      const { count = 1 } = action

      return state + count
    }

    if (isAction<DecrementAction>(action, 'DECREMENT')) {
      // @ts-expect-error
      action.wrongField

      const { count = 1 } = action

      return state - count
    }

    return state
  }

  let s: State = reducer(undefined, { type: 'init' })
  s = reducer(s, { type: 'INCREMENT' })
  s = reducer(s, { type: 'INCREMENT', count: 10 })
  s = reducer(s, { type: 'DECREMENT' })
  s = reducer(s, { type: 'DECREMENT', count: 10 })
  s = reducer(s, { type: 'SOME_OTHER_TYPE', someField: 'value' })

  const combined = combineReducers({ sub: reducer })

  let cs: { sub: State } = combined(undefined, { type: 'init' })
  cs = combined(cs, { type: 'INCREMENT', count: 10 })
}

/**
 * Test ReducersMapObject with default type args.