Question

我一直在尝试访问对象数组中的值，但是由于某些原因，当我使用console.log(quiz.questions[0].question)时出现错误

TypeError：无法读取未定义的属性“问题”

代码：

const quizAPI = process.env.REACT_APP_API_QUIZ;

const PlayQuiz = () => {
  const [quiz, setQuiz] = useState({
    title: '',
    questions: {},
  });
  useEffect(() => {
    async function getQuiz() {
      const param = window.location.search;
      const id = param.split('=');
      const Quiz = await axios.get(`${quizAPI}?_id=${id[1]}`);
      const { doc } = Quiz.data.data;
      console.log(doc[0]);
      const quizDoc = doc[0];
      setQuiz({ ...quiz, title: quizDoc.title, questions: quizDoc.questions });
    }
    getQuiz();
  }, []);
  console.log(quiz.questions[0].question);

console.log(quiz)显示的内容：

{title: "findme", questions: Array(3)}
questions: Array(3)
0: {_id: "5f5500caab44c4bd2b58b109", question: "sfdf", answerSelectionOne: "sdvsv", answerSelectionTwo: "asd", answerSelectionThree: "", …}
1: {_id: "5f5500cdab44c4bd2b58b10a", question: "sfdfddd", answerSelectionOne: "sdvsv", answerSelectionTwo: "asd", answerSelectionThree: "", …}
2: {_id: "5f5500cfab44c4bd2b58b10b", question: "sfdfdddv", answerSelectionOne: "sdvsv", answerSelectionTwo: "asd", answerSelectionThree: "", …}
length: 3
__proto__: Array(0)
title: "findme"
__proto__: Object

Answer 1

通常，这意味着您试图访问可能是null或undefined变量的属性。

可能首先需要更改初始的useState，因为questions是一个数组，而不是对象{}，如下所示：

const [quiz, setQuiz] = useState({
    title: '',
    questions: [],
});

第二quiz.questions可以有length个0。因此，通过像quiz.questions.length > 0这样的简单检查，就可以消除该问题。

然后您可以将日志记录用作：

console.log(quiz.questions.length > 0 ? quiz.questions[0].question : null);

Answer 2

为了正确访问测验字段值，建议使用另一个监视quiz状态的useEffect：

    useEffect(() => {
        
        if(quiz.questions.length){
           console.log(quiz.questions[0].question);
         }
        
    }, [quiz]);

Answer 3

似乎您正在尝试将对象{}用作数组[]！

Javascript试图做的是访问此文件：

{
 "0" : { // your data},
}

应该是什么样子

[
 0 : { // your data},
]

您要更改的是：

const [quiz, setQuiz] = useState({
    title: '',
    questions: [], // make it an array
});

并可能考虑使用.push（）将单个对象推入数组

React.js-无法访问对象数组内的值

3 个答案: