类型“undefined”必须具有返回迭代器的“[Symbol.iterator]()”方法

Question

我正在尝试将我的减速器从 JS 转换为 Typescript，但我遇到了一个奇怪的错误，我无法解决。该错误来自于在减速器[...state.messages]中使用省略号进行数组解构。基本上代码如下：Reducer：

import { HomeScreenAction } from '../actions';
import { HomeState, User, Message } from '../types';
import * as constants from '../constants';
import { combineReducers } from 'redux';

export function userState(state: HomeState = {}, action: 
HomeScreenAction): HomeState {
switch (action.type) {
case constants.REQUEST_USER:
case constants.RECEIVE_USER:
  return { user: action.user };
default:
  return state;
}
}


export function messagesState(state: HomeState = {}, action: 
HomeScreenAction):HomeState {
switch (action.type) {
case constants.REQUEST_MESSAGES:
case constants.RECEIVE_MESSAGES:
  return { messages: action.messages };
case constants.Test1:
  return{ ...state, user: action.payload};
case constants.Test2:
  return {...state, messages:[ ...state.messages, action.payload ]};
default:
  return state;
}
}

const rootReducer = combineReducers({
userState,
messagesState
});

export default rootReducer;

类型：

export interface HomeState {
user?: User;
messages?: Message[];
}
export interface Test1 {
type: typeof constants.Test1
payload: User
}

export interface Test2 {
type: typeof constants.Test2
payload: Message
}

和 tsconfig.js：

  "compilerOptions": {
/* Basic Options */
"target": "es6",                          
"watch": false,
"module": "commonjs",
 "lib":  [ "dom", "es6", "es2015" , "dom.iterable"],
// "allowJs": true,
// "checkJs": true,
"jsx": "react",
// "declaration": true,
"sourceMap": true,
// "outFile": "./",
"outDir": "./lib",

"strict": true

},
"include": ["./src/"]
}

我尝试了不同的库，例如 es2015 和 es5，但仍然收到错误

[...state.messages]

任何帮助将非常感激。

Answer 1

由于messages是 中的可选属性HomeState，因此消息可以是，undefined在这种情况下，数组解构中会出现运行时错误。

为了防止出现运行时错误，TS 会抛出编译时错误。

解决此问题的最简单方法是检查是否state.messages为空并使用空数组进行初始化。例子：

 case constants.Test2:
   let messages: Message[] = state.mesaage || [];
   return {...state, messages:[ ...messages, action.payload ]};