TypeError： undefined 不是对象(评估 'state.favoriteBooks.findIndex')

Question

每次我按收藏按钮，它就会给我一个错误。

TypeError: undefined is not an object  (evaluating 'state.favoriteBooks.findIndex')

这个错误发生在book.js回来的减速机:

import { SET_BOOKS, TOGGLE_FAVORITE } from '../actions/types';
import Book from '../../models/book';
const initialState = {
books: [],
favoriteBooks: [],
};
export default (state = initialState, action) => {
switch (action.type) {

case SET_BOOKS:
return {
books: action.books,
};
case TOGGLE_FAVORITE:
const existingIndex = state.favoriteBooks.findIndex(
(book) => book.id === action.bookId
);
if (existingIndex >= 0) {
const updatedFavBooks = [...state.favoriteBooks];
updatedFavBooks.splice(existingIndex, 1);
return { ...state, favoriteBooks: updatedFavBooks };
} else {
const book = state.books.find((book) => book.id === action.bookId);
return { ...state, favoriteBooks: state.favoriteBooks.concat(book) };
}
default:
return state;
}
};

我认为问题是，它试图找到索引时，变量是空的。但是当我运行调度toggleFavorite(BookId)它在输入书号

Answer 1

在SET_BOOKS返回时我忘记添加...state。

...
case SET_BOOKS:
return {
...state,
books: action.books,
};
...

不要忘记添加...state，因为您想将以前的状态保存在initialState中。这就是为什么它没有在books中拾取任何id。