React - class 组件引用函数组件时出错

Question

我有一个 react/django 应用程序。在我的 app.js 中，我正在测试从我的 API 中提取和呈现 JSON 的能力，但同时能够呈现 React 组件。我的 app.js 包含一个 App class 组件，该组件引用作为功能组件的 Navbar 和 Navitems。我还提到了我在图标文件夹中保存为 word.svg 的图标。我将在开发工具中包含错误消息，在图标文件夹中包含 arrow.svg 的代码，以及我的 App.js 文件夹。

错误：

Target container is not a DOM element.

React.createElement: type is invalid -- expected a string (for built-in components) or a class/function (for composite components) but got: undefined. You likely forgot to export your component from the file it's defined in, or you might have mixed up default and named imports.

Each child in a list should have a unique “key” prop

Element type is invlaid: react-dom.development.js expected a string or class/function but got: undefined

Check the render method of app

The above error occurred in the <a> componenet

App.js

import { render } from "react-dom";
import './index.css';
import { ReactComponent as BellIcon } from './icons/bell.svg';
import { ReactComponent as MessengerIcon } from './icons/messenger.svg';
import { ReactComponent as CaretIcon } from './icons/caret.svg';
import { ReactComponent as PlusIcon } from './icons/plus.svg';
import { ReactComponent as CogIcon } from './icons/cog.svg';
import { ReactComponent as ChevronIcon } from './icons/chevron.svg';
import { ReactComponent as ArrowIcon } from './icons/arrow.svg';
import { ReactComponent as BoltIcon } from './icons/bolt.svg';

import React, { useState, useEffect, useRef, Component } from 'react';
import { CSSTransition } from 'react-transition-group';

class App extends Component {
  constructor(props) {
    super(props);
    this.state = {
      data: [],
      loaded: false,
      placeholder: "Loading"
    };
  }

  componentDidMount() {
    fetch("api/lead")
      .then(response => {
        if (response.status > 400) {
          return this.setState(() => {
            return { placeholder: "Something went wrong!" };
          });
        }
        return response.json();
      })
      .then(data => {
        this.setState(() => {
          return {
            data,
            loaded: true
          };
        });
      });
  }

  render() {
    return (
      <ul>
        {this.state.data.map(contact => {
          return (
            <div>'
              <Navbar>
                <NavItem icon={<PlusIcon />} />
                <NavItem icon={<BellIcon />} />
                <NavItem icon={<MessengerIcon />} />

                <NavItem icon={<CaretIcon />}>
                  <DropdownMenu></DropdownMenu>
                </NavItem>
              </Navbar>
              <li key={contact.id}>
                {contact.name} - {contact.email}
                <h1>Hi</h1>
              </li>
            </div>
          );
        })}
      </ul>
    );
  }
} 

function Navbar(props) {
  return (
    <nav className="navbar">
      <ul className="navbar-nav">{props.children}</ul>
    </nav>
  );
}

function NavItem(props) {
  const [open, setOpen] = useState(false);

  return (
    <li className="nav-item">
      <a href="#" className="icon-button" onClick={() => setOpen(!open)}>
        {props.icon}
      </a>

      {open && props.children}
    </li>
  );
}

function DropdownMenu() {
  const [activeMenu, setActiveMenu] = useState('main');
  const [menuHeight, setMenuHeight] = useState(null);
  const dropdownRef = useRef(null);

  useEffect(() => {
    setMenuHeight(dropdownRef.current?.firstChild.offsetHeight)
  }, [])

  function calcHeight(el) {
    const height = el.offsetHeight;
    setMenuHeight(height);
  }

  function DropdownItem(props) {
    return (
      <a href="#" className="menu-item" onClick={() => props.goToMenu && setActiveMenu(props.goToMenu)}>
        <span className="icon-button">{props.leftIcon}</span>
        {props.children}
        <span className="icon-right">{props.rightIcon}</span>
      </a>
    );
  }

  return (
    <div className="dropdown" style={{ height: menuHeight }} ref={dropdownRef}>

      <CSSTransition
        in={activeMenu === 'main'}
        timeout={500}
        classNames="menu-primary"
        unmountOnExit
        onEnter={calcHeight}>
        <div className="menu">
          <DropdownItem>My Profile</DropdownItem>
          <DropdownItem
            leftIcon={<CogIcon />}
            rightIcon={<ChevronIcon />}
            goToMenu="settings">
            Settings
          </DropdownItem>
          <DropdownItem
            leftIcon=""
            rightIcon={<ChevronIcon />}
            goToMenu="animals">
            Animals
          </DropdownItem>

        </div>
      </CSSTransition>

      <CSSTransition
        in={activeMenu === 'settings'}
        timeout={500}
        classNames="menu-secondary"
        unmountOnExit
        onEnter={calcHeight}>
        <div className="menu">
          <DropdownItem goToMenu="main" leftIcon={<ArrowIcon />}>
            <h2>My Tutorial</h2>
          </DropdownItem>
          <DropdownItem leftIcon={<BoltIcon />}>HTML</DropdownItem>
          <DropdownItem leftIcon={<BoltIcon />}>CSS</DropdownItem>
          <DropdownItem leftIcon={<BoltIcon />}>JavaScript</DropdownItem>
          <DropdownItem leftIcon={<BoltIcon />}>Awesome!</DropdownItem>
        </div>
      </CSSTransition>

      <CSSTransition
        in={activeMenu === 'animals'}
        timeout={500}
        classNames="menu-secondary"
        unmountOnExit
        onEnter={calcHeight}>
        <div className="menu">
          <DropdownItem goToMenu="main" leftIcon={<ArrowIcon />}>
            <h2>Animals</h2>
          </DropdownItem>
          <DropdownItem leftIcon="">Kangaroo</DropdownItem>
          <DropdownItem leftIcon="">Frog</DropdownItem>
          <DropdownItem leftIcon="">Horse?</DropdownItem>
          <DropdownItem leftIcon="">Hedgehog</DropdownItem>
        </div>
      </CSSTransition>
    </div>
  );
}




export default App;

const container = document.getElementById("app");
render(<App />, container);

arrow.svg

    <svg viewBox="0 0 448 512">
        <path fill="currentColor" d="M257.5 445.1l-22.2 22.2c-9.4 9.4-24.6 9.4-33.9 0L7 273c-9.4-9.4-9.4-24.6 0-33.9L201.4 44.7c9.4-9.4 24.6-9.4 33.9 0l22.2 22.2c9.5 9.5 9.3 25-.4 34.3L136.6 216H424c13.3 0 24 10.7 24 24v32c0 13.3-10.7 24-24 24H136.6l120.5 114.8c9.8 9.3 10 24.8.4 34.3z" class=""/>
    </svg>

Answer 1

为封闭的 div 指定 key 属性。键入 li 元素是多余的：

{this.state.data.map((contact, index) => {
  return (
    <div key={contact.id}>'
      <Navbar>
        <NavItem icon={<PlusIcon />} />
        <NavItem icon={<BellIcon />} />
        <NavItem icon={<MessengerIcon />} />

        <NavItem icon={<CaretIcon />}>
          <DropdownMenu></DropdownMenu>
        </NavItem>
      </Navbar>
      <li>
        {contact.name} - {contact.email}
        <h1>Hi</h1>
      </li>
    </div>
  );
})}

React - class 组件引用函数组件时出错

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