nodeJs(Express) server struggled to connect its proxy with view of ReactJs class. and this is the solution https://stackoverflow.com/a/52620241/9422917

package.json(nodejs root)

  "scripts": {
    "client": "npm start --prefix ./client",
    "server": "nodemon server.js",
    "dev": "concurrently  \"npm run server\" \"npm run client\""
    }

server.js(nodejs Root)

var port = 5000;
var express = require('express');
var app = express();


app.listen(port, () => {
    console.log(`\n[Server Start] Port ${port} opened!`)
})

setupProxy.js

const proxy = require('http-proxy-middleware');
module.exports = function (app) {
    app.use(proxy('/', {target: 'http://localhost:5000/'}))
}

Tried to build a React-nodejs(express) app through this guide.
Other advice was not working for me. So some hours wasted.

you need to install 'http-proxy-middleware'

npm i --save http-proxy-middleware

Reference

https://stackoverflow.com/a/52620241/9422917