为什么我的 React-Express 应用程序无法从 express 后端获取和显示数据？

Question

My folder Structure look like this:

The code inside of server.js:

const express = require('express');
const app = express();
const port = process.env.PORT || 5000;

// console.log that your server is up and running
app.listen(port, () => console.log(`Listening on port ${port}`));

// create a GET route
app.get('/express_backend', (req, res) => {
  res.send({ express: 'YOUR EXPRESS BACKEND IS CONNECTED TO REACT' });
});

The code inside of App.js (reside in client folder):

import React, { Component } from 'react';
import logo from './logo.svg';
import './App.css';

class App extends Component {
state = {
    data: null
  };

  componentDidMount() {
      // Call our fetch function below once the component mounts
    this.callBackendAPI().then(res => this.setState({ data: res.express })).catch(err => console.log(err));
  }
    // Fetches our GET route from the Express server. (Note the route we are fetching matches the GET route from server.js
  callBackendAPI = async () => {
    const response = await fetch('/express_backend');
    const body = await response.json();

    if (response.status !== 200) {
      throw Error(body.message) 
    }
    return body;
  };

  render() {
    return (
      <div className="App">
        <header className="App-header">
          <img src={logo} className="App-logo" alt="logo" />
          <h1 className="App-title">Welcome to React</h1>
        </header>
        <p className="App-intro">{this.state.data}</p>
      </div>
    );
  }
}

export default App;

package.json:

{
  "name": "dtdc-inside",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "express": "^4.17.1"
  },
  "proxy": "http://localhost:5000/"
}

I have run "node server.js" without error. cd over to client directory and run "npm start" also without error. But, data from server.js doesn't displayed in the browser. I need help to understand why that happened. Thank you for your attention and forgive my poor english.

Answer 1

const express = require('express');
const app = express();
const port = process.env.PORT || 5000;

//Optional: add this line to parse incoming data. In case you want to accept data
app.use(express.json());

// create a GET route
app.get('/express_backend', (req, res) => {

  //generate output in `json` format rather than `send`.
  res.json({ express: 'YOUR EXPRESS BACKEND IS CONNECTED TO REACT' });
});

//execute your server after writing all logic.
// console.log that your server is up and running
app.listen(port, () => console.log(`Listening on port ${port}`));

Answer 2

Thank you for your attention. With folder structure change (make a new folder called "backend" in the root directory, then move the file "server.js" into inside this folder), the code is now working. Thank you –