Masalah Cors saat melakukan post di server api nodejs memakai express

Saya memiliki kendala saat mengirim data (post) ke server api di nodejs meski menggunakan module cors, namun jika me-retrieve (get) lancar.

Error : 500 internal server error -> 'Allow-origin blabla' is not allowed. (Ini error tentang cors issue).

Spesifikasi : - Menggunakan reactjs (via create-react-app) - Menggunakan express (via express-generator) - Database menggunakan mysql di phpmyadmin

Kode saya :

1. Signup.jsx (Reactjs/clientside)

 import React from 'react';
import ReactDOM from 'react-dom';
import {Container,Form,Button,Input} from 'reactstrap';
import Navigation from 'app/views/templates/Navigation';
import Footer from 'app/views/templates/Footer';
class Signup extends React.Component {
    constructor(){
        super();
        this.state = {
            username: '',
            email: '',
            phone: '',
            village: '',
            district: '',
            province: '',
            country: '',
            zip_code: '',
            password: '',
            confirm_password: '',
            messgae: ''
        }
        this.inputValue = this.inputValue.bind(this);
        this.submitSignup = this.submitSignup.bind(this);
    }
    submitSignup(e){
        e.preventDefault();
        let data = {
            username: this.state.username,
            email: this.state.email,
            phone: this.state.phone,
            village: this.state.village,
            district: this.state.district,
            province: this.state.province,
            country: this.state.country,
            zip_code: this.state.zip_code,
            password: this.state.password,
            confirm_password: this.state.confirm_password
        }
        fetch('/api/users/add',{method:'POST', headers: {'Content-Type': 'application/json'}, body: JSON.stringify(data)})
        .then(function(response){
            if(response.status >= 400){
                console.log(response.status);
            }
            return response;
        })
        .then(function(data){
            if(data == 'success'){
                this.setState({message: 'Thank for registering'});
            }
        });
    }
    inputValue(e) {
        this.setState({[e.target.name]: e.target.value});
    }
	render(){
		const logo = process.env.PUBLIC_URL+'/assets/img/logo.svg';
		return (
			<div>
				<Navigation/>
				<Container fluid style={{marginTop:'58px'}}>
					<br/>
					<Form className="form-responsive" onSubmit={this.submitSignup}>
						<h4 className="blue-light average"><img className="img-md rmargin-sm" src={logo}/>IndoMarket</h4>
						<div className="view tmargin-sm bmargin-sm bg-blue-light"></div>
						<p className="raleway font-lg text-center margin-md">Daftar dengan</p>
						<button type="button" id="oauth-google" className="btn btn-md btn-block btn-default proxima-sb font-md black bmargin-md"><b className="mdi mdi-google-plus font-md rmargin-xs red"></b> Google +</button>
						<hr/>
						<p className="raleway font-lg text-center margin-md">Atau</p>
						<Input type="text" onChange={this.inputValue} name="username" placeholder="Username"/>
						<Input type="email" onChange={this.inputValue} name="email" placeholder="Email"/>
                        <Input type="telp" onChange={this.inputValue} name="phone" placeholder="Phone number"/>
                        <Input type="password" onChange={this.inputValue} name="password" placeholder="Password"/>
                        <Input type="password" onChange={this.inputValue} name="confirm_password" placeholder="Confirm password"/>
						<br/>
						<Button className="bg-blue-light">Submit</Button>
					</Form>
					<br/>
				</Container>
				<Footer/>
			</div>
		);
	}
}
export default Signup;

2. App.js (Config express/serverside)

 var express = require('express'),
http = require('http'),
mysql = require('mysql'),
connection = require('express-myconnection'),
path = require('path'),
cors = require('cors'),
favicon = require('serve-favicon'),
logger = require('morgan'),
cookieParser = require('cookie-parser'),
bodyParser = require('body-parser'),
stylus = require('stylus'),
mysql = require('mysql'),
fs = require('fs'),
ejs = require('ejs'),
routes,
app = express();
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'ejs');
//app.use(favicon(path.join(__dirname, 'public', 'favicon.ico')));
app.use(logger('dev'));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));
app.use(cookieParser());
app.use(stylus.middleware(path.join(__dirname, 'public')));
app.use(express.static(path.join(__dirname, 'public')));
app.use(cors());
fs.readdirSync('./controllers').forEach(function(file){
  if(file.substr(-3) == '.js'){
    routes = require('./controllers/'+file.replace(/.js/,''));
    routes.controller(app);
  }
});
app.use(function(req, res, next) {
  var err = new Error('Not Found');
  err.status = 404;
  next(err);
});
app.use(function(err, req, res, next) {
  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};
  res.status(err.status || 500);
  res.render('error');
});

module.exports = app;

3. UsersModel.js (model saya di express/serverside)

 var connection = require('../bin/connection');
var readUsers = function(res,position,number_page) {
    let limit;
    typeof position !== 'undefined' && typeof number_page !== 'undefined' ? limit = ' LIMIT '+position+','+number_page : limit = '';
    connection.query(" SELECT * FROM users ORDER BY id_user DESC "+limit,function(err,row,field){
        if(err) throw err;
        res.send(JSON.stringify({"status": 200, "error": null, "response": row}));
    });
};
var readUser = function(res) {
    connection.query(" SELECT * FROM users WHERE id_user = ? ",res.id,function(err,row,field){
        if(err) throw err;
        res.send(JSON.stringify({"status": 200, "error": null, "response": row}));
    });
};
var signup = function(res) {
    console.log(res);
    const data = {username: res.body.username, email: res.body.email, phone: res.body.phone, password: res.body.password, confirm_password: res.body.confirm_password}
    connection.query(" INSERT INTO users SET username = '"+res.body.username+"', email = '"+res.body.email+"', phone = '"+res.body.phone+"', password = '"+res.body.password+"', confirm_password = '"+res.body.confirm_password+"'", function(err,row,field){
        if(err) throw err;
        res.send(JSON.stringify({"status": 200, "error": null, "response": row}));
    });
};
module.exports = UsersModel = {
    readUsers: readUsers,
    signup: signup
};

4. index.js (default route/controller saya diexpress/serverside)

 var UsersModel = require('../models/UsersModel'),
cors = require('cors');
var main = function(app){
  app.get('/',function(req,res){
    let data = {
      name: 'Myapp for developer',
      description: 'Building great application for indonesia country state with indomarket Api\'s'
    }
    res.render('index',{title:'Developer',data});
  });
  app.get('/api/users',function(req,res){
    UsersModel.readUsers(res);
  });
  app.post('/api/users/add',function(req,res){
    UsersModel.signup(res);
  });
};
module.exports = { controller: main };

Mohon bantuan dan pertolongannya :-) Terima kasih