GET http://localhost:3000/hello 404（未找到）

我正在遵循實施 MERN 的教程，但我遇到了以下錯誤：GET http://localhost:3000/hello 404 (Not Found)我做的一切都是正確的，但我無法解決這個問題。以下是我的文件，任何人都可以形成社區幫助，以便我繼續前進。我真的很感謝你的幫助。謝謝你們。

前端文件夾：App.js

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

class App extends Component {
   state = {
     hello: null
   }

  componentDidMount() {
      axios.get('/hello')
      .then( res => this.setState({ hello: res.data }))
      .catch( err => console.log(err) )
  
  }

  render() {
     return (
        <div>
          {this.state.hello
            ? <div> {this.state.hello} </div>
            : null }
        </div>
      );
  }
}

export default App;

後端文件夾：App.js

var createError = require('http-errors');
var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser');
var logger = require('morgan');


//import the 'routes' file
var indexRouter = require('./routes')


var app = express();

// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');

app.use(logger('dev'));
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));


// use indexRouter
app.use('/', indexRouter);

module.exports = app;

路線

var express = require('express');

var router = express.Router()  

router.get('/hello', function(req, res) {
    res.json('hello world') ;
})

// this is exported to app.js in the Server folder
module.exports =  router ;

前端文件夾：package.json

...
"eslintConfig": {
    "extends": [
      "react-app",
      "react-app/jest"
    ]
  },
  "proxy": "http://localhost:5000", <==== pointing to localhost
  "browserslist": {
    "production": [
      ">0.2%",
 ...