i got an error: Uncaught (in promise) TypeError: cartItems.map is not a function. The above error occurred in the <CartScreen> component:

Question

我正在学习使用 React 创建完整堆栈 Web 应用程序的 mern 堆栈教程，但我在 cartItems.map-- 未捕获（承诺）TypeError：cartItems.map 不是函数中遇到错误.上面的错误发生在组件中：，我猜是因为一个数组，我试着把它变成一个数组，但它也不适合我

import React, { useEffect } from 'react';
import { useDispatch, useSelector } from 'react-redux';
import { Link, useLocation, useNavigate, useParams } from 'react-router-dom';
import { addToCart } from '../actions/cartActions';
import MessageBox from '../components/MessageBox';

export default function CartScreen(props) {
    const params = useParams();
    const productId = params.id;
    const {search} =useLocation();
    const qtyInUrl = new URLSearchParams(search).get('qty');
    const qty = qtyInUrl?Number(qtyInUrl):1;
  const cart = useSelector(state => state.cart);
  const  {cartItems } = cart;
  const{ error} = cart;
  const dispatch = useDispatch();
  useEffect(() => {
    if (productId) {
      dispatch(addToCart(productId, qty));
    }
  }, [dispatch, productId, qty]);

  const removeFromCartHandler = () => {
    // delete action
    // dispatch(removeFromCart(id));
  };
  const navigate=useNavigate()
  const checkoutHandler = () => {
    navigate('/signin?redirect=shipping');
  };
   return (<div className="row top">
      <div className="col-2">
        <h1>Shopping Cart</h1>
        {error && <MessageBox variant="danger">{error}</MessageBox>}
        {cartItems.length === 0 ? (
          <MessageBox>
            Cart is empty. <Link to="/">Go Shopping</Link>
          </MessageBox>
        ) : (
          <ul>
            {cartItems.map((item) => (
              <li key={item.product}>
                <div className="row">
                  <div>
                    <img
                      src={item.image}
                      alt={item.name}
                      className="small"
                    ></img>
                  </div>
                  <div className="min-30">
                    <Link to={`/product/${item.product}`}>{item.name}</Link>
                  </div>
                  <div>
                    <select
                      value={item.qty}
                      onChange={(e) =>
                        dispatch(
                          addToCart(item.product, Number(e.target.value))
                        )
                      }
                    >
                      {[...Array(item.countInStock).keys()].map((x) => (
                        <option key={x + 1} value={x + 1}>
                          {x + 1}
                        </option>
                      ))}
                    </select>
                  </div>
                  <div>${item.price}</div>
                  <div>
                    <button
                      type="button"
                      onClick={() => removeFromCartHandler(item.product)}
                    >
                      Delete
                    </button>
                  </div>
                </div>
              </li>
            ))}
          </ul>
        )}
      </div>
      <div className="col-1">
        <div className="card card-body">
          <ul>
            <li>
              <h2>
                Subtotal ({cartItems.reduce((a, c) => a + c.qty, 0)} items) : $
                {cartItems.reduce((a, c) => a + c.price * c.qty, 0)}
              </h2>
            </li>
            <li>
              <button
                type="button"
                onClick={checkoutHandler}
                className="primary block"
                disabled={cartItems.length === 0}
              >
                Proceed to Checkout
              </button>
            </li>
          </ul>
        </div>
      </div>
    </div>
  );
}

Answer 1

出现此错误是因为您的数据不是数组。 .map() 函数仅适用于数组。首先，您需要确认您的数据类型。其次，您需要将数据转换为数组。

您应该检查您的 cartItems 类型并更改您的状态以确保有一个数组。

来源：https://developer.mozilla.org/fr/docs/Web/JavaScript/Reference/Global_Objects/Map

i got an error: Uncaught (in promise) TypeError: cartItems.map is not a function. The above error occurred in the <CartScreen> component:

i got an error: Uncaught (in promise) TypeError: cartItems.map is not a function. The above error occurred in the <CartScreen> component:

javascript

arrays

reactjs

react-redux