I am following tutorial for implementing MERN but I got stuck with the error that says: GET http://localhost:3000/hello 404 (Not Found) I did everything correct but I can not get around the problem. Below are my files, Can anyone form the community help so I can move forward. I really appreciate your help. Thank you all.

Frontend Folder: 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;

Backend Folder: 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;

routes

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 ;

Frontend Folder: package.json

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

CodePudding user response：

Ok, i took your code and first issue is that your request is sent to http://localhost:3001/hello - meaning it will take port of your frontend (assume you want to run FE and BE separately, ports 3000 and 3001 for instance)

After i fix axios.get('/hello') with axios.get('http://localhost:3000/hello'), we get to the second issue. I get CORS issue.

In order to fix this we have to configure cors on our express backend -

And third issue is - you don't have app.listen(). app.listen(PORT, () => console.log(`app listening at http://localhost:${PORT}`))

Ok here is the code. First, i removed all extra dependencies for which i thought are not relevant for this case. Then after i make it all working, i put them back, but on some places i might forgot to return all. Also i put your get route directly in index.js - so that differs as well from your code (you put it in separate module). As I say, i tried to keep things simple and easy to grasp.

BE (i run it with node index.js

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

var app = express();
const PORT = 3000;

app.use(cors())
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(express.static(path.join(__dirname, 'public')));
app.use(logger('dev'));
app.use(cookieParser());
 
app.get('/hello', function(req, res) {
    res.send('hello world') ;
})
app.listen(PORT, () => console.log(`app listening at http://localhost:${PORT}`))

module.exports = app;

FE (start it with npm start - it is create-react-app - port 3001, because backend runs on port 3000)

export  class App extends React.Component {
  state = {
    hello: null
  }
 componentDidMount() {
     axios.get('http://localhost:3000/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>
     );
   }
 }