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当我尝试使用 in useEffect 钩子（在组件挂载之前）进行 API 调用时，不知何故状态没有得到更新，因此出现错误Cannot read property of undefined。

但是，如果我将相同的逻辑转换为 Class 组件并在 componentDidMount 函数中进行 API 调用，则代码运行良好。

谁能告诉我为什么？

使用 useEffect

import React from "react";
import axios from "axios";
import { useState, useEffect } from "react";

export default function Customers() {
  const [customers, setCustomers] = useState([]);

  useEffect(() => {
    axios
      .get("http://localhost:5000/customers")
      .then((res) => {
        const data = res.data;
        setCustomers(data);
      })
      .catch((err) => console.log(err));
  }, []);

  useEffect(() => {
    console.log(customers);
  }, [customers]);

  return (
    <div className="container-fluid d-flex flex-column align-items-center justify-content-center">
      <div className="top">Customers</div>
      <div className="tables">
        <table class="table table-striped table-hover">
          <thead>
            <tr>
              <th scope="col">Account No</th>
              <th scope="col">Name</th>
              <th scope="col">E-mail</th>
              <th scope="col">Balance</th>
            </tr>
          </thead>
          <tbody>
            {customers.data.map((customer) => ( // error on this line.
              <tr>
                <th scope="row">{customer.account_no}</th>
                <td>{customer.name}</td>
                <td>{customer.email}</td>
                <td>{customer.balance}</td>
              </tr>
            ))}
          </tbody>
        </table>
      </div>
    </div>
  );
}

基于类的组件

import React, { Component } from "react";
import axios from "axios";
import "./Customers.css";

export default class Customers extends Component {
  state = {
    customers: [],
  };

  componentDidMount() {
    axios
      .get("http://localhost:5000/customers")
      .then((res) => {
        res.data.sort();
        console.log(res.data);
        this.setState({ customers: res.data });
      })
      .catch((err) => console.log(err));
  }

  render() {
    return (
      <div className="container-fluid main w-75 my-4 d-flex flex-column align-items-center">
        <div className="top p-4 d-flex justify-content-center">
          Our Customers
        </div>
        <div className="tables w-100">
          <table class="table table-striped table-hover">
            <thead>
              <tr>
                <th scope="col">Account No</th>
                <th scope="col">Name</th>
                <th scope="col">E-mail</th>
                <th scope="col">Balance</th>
              </tr>
            </thead>
            <tbody>
              {this.state.customers.map((customer) => (
                <tr>
                  <th scope="row">{customer.account_no}</th>
                  <td>{customer.name}</td>
                  <td>{customer.email}</td>
                  <td>{customer.balance}</td>
                </tr>
              ))}
            </tbody>
          </table>
        </div>
      </div>
    );
  }
}

4

2 回答 2

3

您没有在useEffect钩子中正确设置状态。而不是setCustomers({data:data});应该只是setCustomers(data);

useEffect(() => {
    axios
      .get("http://localhost:5000/customers")
      .then((res) => {
        const data = res.data;
        setCustomers(data);
      })
      .catch((err) => console.log(err));
  }, []);

现在因为customers是一个数组，只需映射customers而不是customers.data.map.

customers.map((customer)=>{})

所以最终的代码将是

import React from "react";
import axios from "axios";
import { useState, useEffect } from "react";

export default function Customers() {
  const [customers, setCustomers] = useState([]);

  useEffect(() => {
    axios
      .get("http://localhost:5000/customers")
      .then((res) => {
        const data = res.data;
        setCustomers(data);
      })
      .catch((err) => console.log(err));
  }, []);

  useEffect(() => {
    console.log(customers);
  }, [customers]);

  return (
    <div className="container-fluid d-flex flex-column align-items-center justify-content-center">
      <div className="top">Customers</div>
      <div className="tables">
        <table class="table table-striped table-hover">
          <thead>
            <tr>
              <th scope="col">Account No</th>
              <th scope="col">Name</th>
              <th scope="col">E-mail</th>
              <th scope="col">Balance</th>
            </tr>
          </thead>
          <tbody>
            {customers.map((customer) => ( 
              <tr>
                <th scope="row">{customer.account_no}</th>
                <td>{customer.name}</td>
                <td>{customer.email}</td>
                <td>{customer.balance}</td>
              </tr>
            ))}
          </tbody>
        </table>
      </div>
    </div>
  );
}
于 2021-06-23T12:51:17.017 回答
2

您将客户状态声明为数组：

const [customers, setCustomers] = useState([]);

但是您在获取数据后传递了一个对象：

 setCustomers({ data: data });

这就是返回部分中的映射迭代失败的原因，因为您将状态设置为对象而不是数组。如果 data 是一个数组，你应该只像这样分配它：

setCustomers(data);

componentDidMount 有效，因为您将 res.data 直接分配给客户状态，结果类似于：

setCustomers(data);
于 2021-06-23T12:52:34.573 回答