react-web-tutorial/back/index.js

import express from 'express';
import bodyParser from 'body-parser';
import cors from 'cors';

const app = express();

const whitelist = ['http://localhost:8000'];
const corsOptions = {
  origin: (origin, callback) => {
    if (whitelist.indexOf(origin) !== -1) {
      callback(null, true)
    } else {
      callback(new Error('Not allowed by CORS'))
    }
  }
};
app.use(cors(corsOptions));
app.use(express.static('public'));

app.listen(5000);