How to fix "TypeError: results.map is not a function", in React

Question

function Results(props) {
  var results = props.results;
  return (
    <>
      <table>
        <thead>
          <tr>
            <th>Book Name</th>
            <th>Author</th>
            <th>S.no</th>
            <th>Series Name</th>
            <th>Type</th>
            <th>Genre</th>
            <th>Kindle/Real</th>
          </tr>
        </thead>
        <tbody>
          {results.map(result => {
            return (
              <tr key={result.name}>
                <td>{result.name}</td>
                <td>{result.author}</td>
                <td>{result.sno}</td>
                <td>{result.series}</td>
                <td>{result.type}</td>
                <td>{result.genre}</td>
                <td>{result.kindeReal}</td>
              </tr>
            );
          })}
        </tbody>
      </table>
    </>
  );
}

当我尝试呈现上面的 com 组件时，出现错误：

TypeError: results.map is not a function

结果变量是一个对象数组，类似于：

[{"type":1},{"type":0},{"type":2}]

但是，当我使用.map函数时，returns报错了！明明是数组，为什么我不能用？

这是console.log（结果）的输出。

[{"Book_Name":"Time Riders","Author":"Alex Scarrow","S_no":1,"Series_Name":"Time Riders","Fiction_Non_fiction_Companion_Prequel":"Fiction","Genre":"Sci-fi/Mystery/Thriller","Kindle_Real":"Real"},{"Book_Name":"Day of the Predator ","Author":"Alex Scarrow","S_no":2,"Series_Name":"Time Riders","Fiction_Non_fiction_Companion_Prequel":"Fiction","Genre":"Sci-fi/Mystery/Thriller","Kindle_Real":"Real"},{"Book_Name":"The Doomsday Code","Author":"Alex Scarrow","S_no":3,"Series_Name":"Time Riders","Fiction_Non_fiction_Companion_Prequel":"Fiction","Genre":"Sci-fi/Mystery/Thriller","Kindle_Real":"Real"},{"Book_Name":"The Eternal War","Author":"Alex Scarrow","S_no":4,"Series_Name":"Time Riders","Fiction_Non_fiction_Companion_Prequel":"Fiction","Genre":"Sci-fi/Mystery/Thriller","Kindle_Real":"Real"},{"Book_Name":"Gates of Rome","Author":"Alex Scarrow","S_no":5,"Series_Name":"Time Riders","Fiction_Non_fiction_Companion_Prequel":"Fiction","Genre":"Sci-fi/Mystery/Thriller","Kindle_Real":"Real"},{"Book_Name":"City of Shadows","Author":"Alex Scarrow","S_no":6,"Series_Name":"Time Riders","Fiction_Non_fiction_Companion_Prequel":"Fiction","Genre":"Sci-fi/Mystery/Thriller","Kindle_Real":"Real"},{"Book_Name":"The Pirate Kings","Author":"Alex Scarrow","S_no":7,"Series_Name":"Time Riders","Fiction_Non_fiction_Companion_Prequel":"Fiction","Genre":"Sci-fi/Mystery/Thriller","Kindle_Real":"Real"},{"Book_Name":"The Mayan Prophecy","Author":"Alex Scarrow","S_no":8,"Series_Name":"Time Riders","Fiction_Non_fiction_Companion_Prequel":"Fiction","Genre":"Sci-fi/Mystery/Thriller","Kindle_Real":"Real"},{"Book_Name":"The Infinity Cage","Author":"Alex Scarrow","S_no":9,"Series_Name":"Time Riders","Fiction_Non_fiction_Companion_Prequel":"Fiction","Genre":"Sci-fi/Mystery/Thriller","Kindle_Real":"Real"}]

我觉得它像一个数组。那为什么不是数组呢？

这是父组件。

import React from "react";
import Results from "./results";

function ResultsRenderer(props) {
  if (props.status === true) {
    return <Results results={props.results} />;
  } else {
    return <>{"No"}</>;
  }
}

export default ResultsRenderer;

这是 ResultsRenderer 的父组件。

import React, { useState } from "react";
import { useEffect } from "react";
import "bootstrap/dist/css/bootstrap.min.css";
import Form from "./searcherFormDumb";
import { toast } from "react-toastify";
import ResultsRenderer from "./resultsRenderer";

function Searcher() {
  const [answer, setAnswer] = useState(["Empty", false]);
  const [book, setBook] = useState({
    name: "",
    author: "",
    sno: null,
    series: "",
    type: "",
    genre: "",
    kindleReal: ""
  });
  const defaultState = {
    name: "",
    author: "",
    sno: null,
    series: "",
    type: "",
    genre: "",
    kindleReal: ""
  };

  function handleChange(event) {
    const updatedBook = { ...book, [event.target.name]: event.target.value };
    setBook(updatedBook);
  }

  function handleSubmit(event) {
    event.preventDefault();
    var changed = {};
    function populateChanged(now, old, title, temp) {
      if (now !== old) {
        temp[title] = now;
        return temp;
      } else {
        return temp;
      }
    }
    changed = populateChanged(
      book.name,
      defaultState.name,
      "Book_Name",
      changed
    );
    changed = populateChanged(
      book.author,
      defaultState.author,
      "Author",
      changed
    );
    changed = populateChanged(book.sno, defaultState.sno, "S_no", changed);
    changed = populateChanged(
      book.series,
      defaultState.series,
      "Series_Name",
      changed
    );
    changed = populateChanged(
      book.type,
      defaultState.type,
      "Fiction_Non_fiction_Companion_Prequel",
      changed
    );
    changed = populateChanged(book.genre, defaultState.genre, "Genre", changed);
    changed = populateChanged(
      book.kindleReal,
      defaultState.kindleReal,
      "Kindle_Real",
      changed
    );
    var temp_string = "";
    var key = "";
    var value = "";
    var temp_string_list = [];
    //debugger;
    for (var i = 0; i < Object.keys(changed).length; i++) {
      //debugger;
      key = Object.keys(changed)[i];
      value = changed[key];
      if (i !== Object.keys(changed).length - 1) {
        temp_string = `${key} = "${value}" AND `;
      } else if (i === Object.keys(changed).length - 1) {
        temp_string = `${key} = "${value}"`;
      }
      temp_string_list.push(temp_string);
      //debugger;
      temp_string = "";
      key = "";
      value = "";
    }

    var sql_t = temp_string_list.join("");
    var sql_tt = "SELECT * FROM books_catalouge WHERE ";
    var sql = sql_tt + sql_t;
    toast.success(sql);

    var request = new XMLHttpRequest();
    var jsql = JSON.stringify(sql);
    request.onreadystatechange = function() {
      //debugger;
      if (this.readyState == 4 && this.status == 200) {
        setAnswer([this.responseText, true]);
        console.log(`${answer}`);
      }
    };
    request.open(
      "GET",
      "http://localhost:3001/retrieve_books" + "?msg=" + jsql,
      true
    );
    request.send(jsql);
    console.log("This is the END");
    console.log(`${answer}`);
  }

  return (
    <>
      <Form book={book} onChange={handleChange} onSubmit={handleSubmit} />
      <br />
      <ResultsRenderer status={answer[1]} results={answer[0]} />
    </>
  );
}

export default Searcher;

如果您也需要 NodeJS，请告诉我。我正在使用 SQL 来获取数据，这就是我需要 NodeJS 的原因。对不起，如果我的代码有点奇怪。

提前致谢！

Answer 1

函数 map() 只能用于数组。在这种情况下，看起来 props.results 不是数组或尚未设置（如果您使用 Axios 或类似的东西获取数据，可能会发生这种情况）。

我建议你在函数的开头放置这样的东西：

if (!props.results) return 'no data';
if (!Array.isArray(props.results)) return 'results are not array'

澄清后

您的 onreadystatechange 请求通常会在 JSON 或 XML 中得到响应。我认为你的答案是字符串化的 json。尝试在 JSON 的情况下使用 JSON.parse(response) 或在 XML 的情况下使用 (new DOMParser()).parseFromString(response,"text/xml")。

在你的组件中它可能看起来像这样

request.onreadystatechange = function() {
   (this.readyState == 4 && this.status == 200)
      && setAnswer([JSON.parse(this.responseText), true]);
};

Answer 2

我在访问数组的数据元素时也遇到了同样的错误。这是我所做的：

{standardObj ? (
    {standardObj.map((obj: any) => {
      return (
        <Grid item>
          <FormControlLabel
            className={styles.CheckboxLabel}
            key={obj.Code}
            control={
              <Checkbox
                onChange={handleCheckElement}
                name={obj.FullName}
                value={obj.Code}
              />
            }
            label={obj.FullName}
          />
        </Grid>
      );
    })}
) : null }

Answer 3

我在反序列化一个简单的 .json 文件时看到了类似的行为：

const stringifiedData = fs.readFileSync(animalsPath);
const animalData = JSON.parse(stringifiedData);
const animals = animalData.map(animal => {
    return new Animal(animal)
})

奇怪的是，我下面的例子是做一个字符串点符号，它似乎满足 V8 对象是一个数组，即使它在 console.out 中的输出没有改变。以下确实有效，但我不知道为什么。有什么想法吗？

const stringifiedData = fs.readFileSync(animalsPath);
const animalData = JSON.parse(stringifiedData).animals;
const animals = animalData.map(animal => {
    return new Animal(animal)
})

Answer 4

我遇到了同样的问题，并且认为对于 useState Hook 来说，初始值的数据类型非常重要，我有一个 0 int 作为初始值，并且反应的 cos 没有将更新识别为数组。

Answer 5

大多数时候这个问题是因为语法错误。

哪里有

 results.map(result => {
 ........................
 }

就这样

results.map(result => (
 ........................
 )

Answer 6

我是 React 的新手，发现如果变量为空/未定义并与“map”函数一起使用，React 会抛出该异常。我的解决方案是检查变量是否为“未定义”且长度 > 0。

How to fix "TypeError: results.map is not a function", in React

How to fix "TypeError: results.map is not a function", in React

javascript

arrays

reactjs

object

array.prototype.map

澄清后