html - 错误 : Element type is invalid: expected a string (for built-in components) or a class/function

Question

• 我正在尝试学习 react redux api 调用
• 所以我举了一个例子并在 stackblitz 中实现，但是当我选择某个 channel 并点击热门新闻按钮时出现以下错误
• 你能告诉我怎么解决吗
• 在下面提供我的代码和 stackblitz https://medium.com/@lavitr01051977/basic-react-redux-app-with-async-call-to-api-e478e6e0c48b

https://stackblitz.com/edit/react-redux-realworld-j95tpu?file=actions/index.js

code
export function fetchPosts(channel) {
  return function (dispatch) {
    dispatch(requestPosts());
    return fetch(`https://newsapi.org/v1/articles?source=${channel}&apiKey=${MY_API_KEY}`)
      .then(response => response.json(),
      error => console.log('An error occurred.', error),
    )
      .then((json) => {
        dispatch(receivedPosts(json));
      }, );
  };
}

error

VM1672:37 Warning: 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. Check the render method of `TopNews`.
    in TopNews (created by Connect(TopNews))
    in Connect(TopNews) (created by App)
    in div (created by App)
    in App
    in Provider


Uncaught (in promise) Error: Element 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. Check the render method of `TopNews`.
    at invariant (invariant.js:42)
    at instantiateReactComponent (instantiateReactComponent.js:74)
    at instantiateChild (ReactChildReconciler.js:44)
    at eval (ReactChildReconciler.js:71)
    at traverseAllChildrenImpl (traverseAllChildren.js:77)
    at traverseAllChildren (traverseAllChildren.js:172)
    at Object.instantiateChildren (ReactChildReconciler.js:70)
    at ReactDOMComponent._reconcilerInstantiateChildren (ReactMultiChild.js:185)
    at ReactDOMComponent.mountChildren (ReactMultiChild.js:224)
    at ReactDOMComponent._createInitialChildren (ReactDOMComponent.js:703)

Answer 1

实际问题是导入/导出语法问题。我将向您介绍如何找到它。

问题的真实位置由组件堆栈跟踪显示:

Warning: 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. Check the render method of `TopNews`.
    in TopNews (created by Connect(TopNews))
    in Connect(TopNews) (created by App)
    in div (created by App)
    in App
    in Provider

如果我们查看 TopNews.js , 它只呈现 <h3> , <div> , 和 <NewsItem> . h3和 div只是基本标签，所以它可能是NewsItem这就是问题所在。

在 TopNews.js 的顶部，你有:

import { NewsItem } from '../components/NewsItem';

然而，在NewsItem.js ，你有:

export default NewsItem ;

因此，您进行的是默认导出，但进行的是命名导入，所以 NewsItem在 TopNews.js 中未定义.

尝试将其更改为 import NewsItem from "../components/NewsItem"它应该有效。