为什么要添加扩展运算符使代码返回空数组?

Question

现在，我正在使用 react 和 redux 创建一个书籍跟踪应用程序，我需要异步获取数据，现在我有 3 个版本的代码 其中两个返回预期结果，但第三个不返回

第一个：

import { FETCH_BOOKS } from '../actions/Types.js';
import * as BooksAPI from '../BooksAPI'
export const bookReducer = (state = [], action) => {
switch(action.type) {
case FETCH_BOOKS:
console.log (BooksAPI.getAll().then(books => books))
default: 
return state;
}
}
//Promise {<pending>}
//this returns a promise in a pending state that is resolved to array of objects(expected)

第二个：

import { FETCH_BOOKS } from '../actions/Types.js';
import * as BooksAPI from '../BooksAPI'
export const bookReducer = (state = [], action) => {
switch(action.type) {
case FETCH_BOOKS:
console.log ([BooksAPI.getAll().then(books => books)])
default: 
return state;
}
}
//[promise]
//this returns an array of a promise that is resolved to array of another array of books objects(expected)

第三个：

import { FETCH_BOOKS } from '../actions/Types.js';
import * as BooksAPI from '../BooksAPI'
export const bookReducer = (state = [], action) => {
switch(action.type) {
case FETCH_BOOKS:
console.log ([...BooksAPI.getAll().then(books => books)])
default: 
return state;
}
}
//[]
//this returns an empty array, no promise
//expected spreading items of the returned array of objects not empty array!!

那么这里有什么问题？？

Answer 1

这不是对你问题的直接回答，但我想指出化简器需要同步。如果你想用redux做异步的事情，你需要包含一个异步中间件。一些流行的选项是：

• Redux-thunk(基于承诺(
• redux-saga(基于生成器(
• redux-observable(基于可观察量(