演示

Question

React-TypeError：无法读取未定义的属性“img”

2021-07-15

182

reactjs react-hooks undefined typeerror

我正在使用 React 制作纸牌游戏，由于某种原因，我得到了 TypeError: Cannot read property 'img' of undefined 。

导致错误的代码：

import React, { useState } from "react";

import Card from "./components/Card";

const App = () => {
    const [cards, setCards] = useState([
        {
            name: "King",
            img: "https://image.freepik.com/free-vector/king-crown-esport-logo-design_177315-269.jpg",
            pts: 10000,
        },
        {
            name: "Minister",
            img: "https://i.pinimg.com/474x/51/59/b3/5159b321713edef6c6a3b8f930f667a1.jpg",
            pts: 5000,
        },
        {
            name: "Cop",
            img: "https://image.shutterstock.com/image-vector/police-officer-badge-icon-vector-260nw-613493573.jpg",
            pts: 100,
        },
        {
            name: "Thief",
            img: "https://images1.livehindustan.com/uploadimage/library/2019/05/03/16_9/16_9_1/thief_symbolic_photo__1556892712.jpg",
            pts: 0,
        },
    ]);

    const [shuffledCards, setShuffledCards] = useState([]);

    for (let i = 0; i < 4; i++) {
        const randCard = cards[Math.floor(Math.random() * (cards.length + 1))];
        setShuffledCards([...shuffledCards, randCard]);
        setCards(cards.filter((c) => c !== randCard));
    }

    return (
        <div style={{ display: "flex", flexDirection: "row" }}>\
            {shuffledCards.map((c) => (
                <Card img={c.img} name={c.name} pts={c.pts} />
            ))}
        </div>
    );
};

export default App;

项目链接（在线 Gitpod.io） - https://purple-salmon-uy3kza3n.ws-us10.gitpod.io/?folder=vscode-remote%3A%2F%2Fsapphire-quokka-1i5clgk7.ws-us09.gitpod.io%3A443%2Fworkspace （注意它可能会改变，一些奇怪的 gitpod 东西）

现在，当我在 for 循环中打印 shuffledCards 数组时，经过几次循环后，我注意到一个未定义的对象被添加到数组中。我认为这就是问题所在，我不知道为什么或如何发生，也不知道如何解决它。

期待收到你们的来信。提前谢谢大家。

Answer 1

主要问题是如何计算随机卡片，您正在从数组中编制索引。

const randCard = cards[Math.floor(Math.random() * (cards.length + 1))];

您想使用长度。

const randCard = cards[Math.floor(Math.random() * (cards.length))];

另一个问题是，您正在无条件地从组件主体更新状态，这将导致渲染循环。

将循环放在 useEffect 钩子中并使用功能状态更新，因为您正在循环中排队状态更新。

useEffect(() => {
  for (let i = 0; i < 4; i++) {
    const randCard = cards[Math.floor(Math.random() * (cards.length))];
    setShuffledCards((shuffledCards) => [...shuffledCards, randCard]);
    setCards((cards) => cards.filter((c) => c !== randCard));
  }
}, []);

存储两个数组是多余的。我会研究并实施 fisher-yates 改组。

这是一个示例效果：

useEffect(() => {
  setCards((cards) => {
    // create copy
    const shuffledCards = cards.slice();

    // shuffle
    for (let i = cards.length - 1; i >= 0; i--) {
      const randIndex = Math.floor(Math.random() * i);
      [shuffledCards[i], shuffledCards[randIndex]] = [
        shuffledCards[randIndex],
        shuffledCards[i]
      ];
    }

    // return next state
    return shuffledCards;
  });
}, [shuffledCards]);

演示

在此演示中，我添加了一个按钮来播放随机播放，这会更新 useEffect 依赖项以触发另一次播放。此效果依赖项可以是任何您需要触发它的内容，以满足您的特定用例。

演示代码

const App = () => {
  const [cards, setCards] = useState([
    {
      name: "King",
      img:
        "https://image.freepik.com/free-vector/king-crown-esport-logo-design_177315-269.jpg",
      pts: 10000
    },
    {
      name: "Minister",
      img:
        "https://i.pinimg.com/474x/51/59/b3/5159b321713edef6c6a3b8f930f667a1.jpg",
      pts: 5000
    },
    {
      name: "Cop",
      img:
        "https://image.shutterstock.com/image-vector/police-officer-badge-icon-vector-260nw-613493573.jpg",
      pts: 100
    },
    {
      name: "Thief",
      img:
        "https://images1.livehindustan.com/uploadimage/library/2019/05/03/16_9/16_9_1/thief_symbolic_photo__1556892712.jpg",
      pts: 0
    }
  ]);

  const [shuffledCards, setShuffledCards] = useState(true);
  const shuffle = () => setShuffledCards((t) => !t);

  useEffect(() => {
    setCards((cards) => {
      const shuffledCards = cards.slice();

      for (let i = cards.length - 1; i >= 0; i--) {
        const randIndex = Math.floor(Math.random() * i);
        [shuffledCards[i], shuffledCards[randIndex]] = [
          shuffledCards[randIndex],
          shuffledCards[i]
        ];
      }
      return shuffledCards;
    });
  }, [shuffledCards]);

  return (
    <>
      <div style={{ display: "flex", flexDirection: "row" }}>
        {cards.map((c) => (
          <Card key={c.name} img={c.img} name={c.name} pts={c.pts} />
        ))}
      </div>
      <button type="button" onClick={shuffle}>
        Shuffle Cards
      </button>
    </>
  );
};