为什么TypeScript无法检查函数的返回类型

我有一个应该返回的函数NetworkState; 但是，从代码中可以看出，在大多数情况下，该函数未返回正确的类型，但TypeScript没有给出错误。我想念什么？

第二次更新：谢谢大家的投入。我已尽可能地复制和简化了代码，完整的代码已复制并粘贴到TypeScript游乐场。不幸的是，我必须包括所有类型才能重现该问题，因此代码比我希望的要长一点。请查看我认为应该有错误（由于返回类型）而没有错误的最后一行。

非常感谢您的任何帮助，谢谢。

更新和说明：

范例1：

const someFunction = (): string => 45

上面函数的返回类型为，string但是我们正在返回一个数字，因此TypeScript给出了一个错误：Type '45' is not assignable to type 'string'

范例2：

type MyType = {
    [key: string]: string | undefined
}

const someFunction = (): MyType => 45

上面函数的返回类型为，MyType但是我们正在返回一个数字，因此TypeScript给出了一个错误：Type '45' is not assignable to type 'MyType'

以下代码的问题：

在下面的示例中，networkStateReducer预期返回NetworkState类型。但是，即使它返回的数据不符合NetworkState，也仍然没有错误。

例如，如果我们仔细观察第一个case：

case NetworkActionType.Failure:
      return { ...state, [action.payload.sagaAction]: 'SHOULD_BE_ERROR' }

假设state最初是空对象，则返回值为：[action.payload.sagaAction]等于某个字符串，但签名清楚地将其设置为对象：

  [key in NetworkSagaAction]?: {
    networkError?: Error
    networkStatus: NetworkStatus
  }

但是，我们没有从TypeScript得到任何错误。

实际代码：

export type NetworkSagaAction = 'Login' | 'Logout'

export type NetworkStatus = 'idle' | 'pending' | 'failure' | 'success'

export enum NetworkActionType {
  Busy = 'Network/Busy',
  Failure = 'Network/Failure',
  Idle = 'Network/Idle',
  Reset = 'Network/Reset',
  Success = 'Network/Success',
}

export type NetworkState = {
  [key in NetworkSagaAction]?: {
    error?: Error
    networkStatus: NetworkStatus
  }
}

export interface NetworkPayload {
  sagaAction: NetworkSagaAction
}

export const initialState: NetworkState = {}

type FunctionType = (...args: any[]) => any

interface ActionCreatorsMapObject {
  [actionCreator: string]: FunctionType
}

export type ActionsUnion<A extends ActionCreatorsMapObject> = ReturnType<A[keyof A]>

export interface Action<T extends string> {
  type: T
}

export interface ActionWithPayload<T extends string, P> extends Action<T> {
  payload: P
}

export type BusyAction = ActionWithPayload<NetworkActionType.Busy, NetworkPayload>

export function createAction<T extends string, P>(type: T, payload: P): ActionWithPayload<T, P>
export function createAction<T extends string, P>(type: T, payload?: P) {
  return payload === undefined ? { type } : { type, payload }
}

export type NetworkActions = ActionsUnion<typeof NetworkActions>
export const NetworkActions = {
    busy: (payload: NetworkPayload): BusyAction => createAction(NetworkActionType.Busy, payload),
}

const networkStateReducer = (
    state = initialState,
    action: NetworkActions,
): NetworkState => { 
    return {
        [action.payload.sagaAction]: 'THIS SHOULD BE OBJECT BUT NOT AND STILL NO TYPE ERROR'
    }
}