Question

返回错误：./src/index.js 尝试导入错误：“ ./ App”不包含默认导出（导入为“ App”）。

import React, { Component, useState } from "react";

const App = () => {
    const [count, setCount] = useState(0);

  const  increment = () => {
        setCount(count + 1);
    };

    return (
      <div>
          <h2> counter app </h2>
          <button onClick={increment}>Clicked {count} times</button>
      </div>
    );
};

索引

import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';


ReactDOM.render(<App />, document.getElementById('root'));

Answer 1

在Nodejs中，要在另一个文件中使用变量或函数，必须将其导出。我们有两种类型的导出。

使用导出常量

// Export a variable
export const App = () => { ... }

// Import App in another file
import { App } from '...'

使用导出默认值

// Export default
const App = () => { ... }
export default App

// Import App in another file
import App from '...'

按照我的示例查看您的代码。您缺少导出App才能在另一个文件中使用此变量。

因此，在您的情况下，您必须导出App以便在index.js中使用：

import React, { Component, useState } from "react";

const App = () => {
    const [count, setCount] = useState(0);

  const  increment = () => {
        setCount(count + 1);
    };

    return (
      <div>
          <h2> counter app </h2>
          <button onClick={increment}>Clicked {count} times</button>
      </div>
    );
};
export default App

请记住，您在一个文件中只有一个导出默认值。

Answer 2

import React, { Component, useState } from "react";

const App = () => {
    const [count, setCount] = useState(0);

  const  increment = () => {
        setCount(count + 1);
    };

    return (
      <div>
          <h2> counter app </h2>
          <button onClick={increment}>Clicked {count} times</button>
      </div>
    );
};

export default App;

Answer 3

import openSocket from 'socket.io-client';

class Socket {
  constructor() {
    this.socket = openSocket('http://localhost:8080');
    this.socket.emit("connection", 1000);
  }
}

export default new Socket();


import socketService  from "../../services/socketService";

socketService.socket.on("new-order", (result) => {
  if (result.data) {
    console.log('order page',result.data);
    let x = [...records];
    x.unshift(result.data);
    setRecords(x);
  }
});

反应-错误：尝试导入错误/“应用程序”不包含默认导出（作为“应用程序”导入

3 个答案: