Question

我是Typescript的新手，正在将我的本机代码库迁移到Typescript。我尝试解决一个奇怪的错误，但无法解决。这是减速器：

const INITIAL_STATE: MessagingState = {
conversations: [],
send: [],
read: []
};

export default function (state: MessagingState = INITIAL_STATE, action: 
UserMessagingActionTypes): MessagingState {
switch (action.type) {
    case USER_CONVERSATIONS_RECEIVED:
        return {...state, conversations: action.payload};
    case USER_CONVERSATION_RECEIVED:
        return {
            ...state,
            conversations: [...state.conversations, action.payload]
        };
    default:
        return state;
   }
}

export interface MessagingState {
conversations: Conversation[]
send: StompMessage[]
read: ChatMessage[]
}

和操作类型：

export interface UserConversationsReceived {
type: typeof USER_CONVERSATIONS_RECEIVED,
payload: Conversation[]
}

export interface UserConversationReceived {
type: typeof USER_CONVERSATION_RECEIVED,
payload: Conversation
}

export interface UserStompUnseenMessageReceived  {
type: typeof MESSAGING_ACTIONS.USER.UNSEEN_MESSAGE_RECEIVED,
payload: ChatMessage
}


export const MESSAGING_ACTIONS = {
  USER:{UNSEEN_MESSAGE_RECEIVED: 'UNSEEN_MESSAGE_RECEIVED'}
}
export const USER_CONVERSATION_RECEIVED= 'USER_CONVERSATION_RECEIVED';
export const USER_CONVERSATIONS_RECEIVED = 'USER_CONVERSATIONS_RECEIVED';

export type UserMessagingActionTypes =
UserConversationsReceived
| UserConversationReceived
| UserStompUnseenMessageReceived;

我原本希望在减速器上使用类型变窄，但是在减速器的return语句中出现以下两个错误：

错误：（14，31）TS2322：键入“对话| ChatMessage |不能将“ Conversation []”分配给“ Conversation []”类型。类型“对话”缺少类型“对话[]”中的以下属性：长度，弹出，推入，连续和另外26个。

错误：（18，17）TS2322：类型'（Conversation | ChatMessage | Conversation []）[]'无法分配给类型'Conversation []'。输入“会话| ChatMessage |不能将“ Conversation []”分配给“ Conversation”类型。类型“ ChatMessage”缺少类型“对话”中的以下属性：标题，unreadMessages，消息

为什么不能将Conversaion []分配给Conversation []？

修改1： 这是罪魁祸首：

export const MESSAGING_ACTIONS = {
  USER:
   {UNSEEN_MESSAGE_RECEIVED: 'UNSEEN_MESSAGE_RECEIVED'}
}

当我将其更改为这样的字符串常量时：

export const UNSEEN_MESSAGE_RECEIVED: 'UNSEEN_MESSAGE_RECEIVED';

问题已解决。我不知道两者之间的 typeof 有何不同。它们都是字符串。但是我不知道编译器怎么了。

Answer 1

我不太确定为什么TypeScript无法根据您的情况缩小类型。我在TypeScript Playground.

上工作了

请注意，您的操作的类型必须是立即为其赋值的字符串，这些字符串要么没有类型定义export const USER_CONVERSATION_RECEIVED = 'USER_CONVERSATION_RECEIVED'，要么具有特定的字符串作为类型定义，例如export const USER_CONVERSATION_RECEIVED: 'USER_CONVERSATION_RECEIVED' = 'USER_CONVERSATION_RECEIVED'。

export const USER_CONVERSATION_RECEIVED = 'USER_CONVERSATION_RECEIVED'; // Correct
export const USER_CONVERSATION_RECEIVED: 'USER_CONVERSATION_RECEIVED' = 'USER_CONVERSATION_RECEIVED'; // Correct
export const USER_CONVERSATION_RECEIVED: string = 'USER_CONVERSATION_RECEIVED'; // Wrong

似乎TypeScript推断您的嵌套属性为字符串类型（如您所知）。如果在原始类型字符串上使用const，则永远不能更改该值-因此，可以将类型从字符串缩小为“ UNSEEN_MESSAGE_RECEIVED”。如果将其嵌套在对象内，则对象引用本身始终是相同的-但您可以更改该对象的属性。因此，TypeScript必须假定类型为字符串（并且不能进一步缩小类型），因为它可以在某个时候更改（因为默认情况下它不是只读的）。

很明显，一种解决方法是不嵌套它。或者，您可以为Typescript提供更窄的类型，尽管看起来很冗长：

export const MESSAGING_ACTIONS: { 
    USER: {
        UNSEEN_MESSAGE_RECEIVED: 'UNSEEN_MESSAGE_RECEIVED'
    }
} = {
    USER:{
        UNSEEN_MESSAGE_RECEIVED: 'UNSEEN_MESSAGE_RECEIVED'
    }
}

// or with an interface

export interface MessagingActions {
    USER: {
        UNSEEN_MESSAGE_RECEIVED: 'UNSEEN_MESSAGE_RECEIVED'
    }
}

export const MESSAGING_ACTIONS:MessagingActions = {
    USER:{
        UNSEEN_MESSAGE_RECEIVED: 'UNSEEN_MESSAGE_RECEIVED'
    }
}

TS2322类型X | Y | Z []无法分配给Z []类型-Typescript

1 个答案: