How to update a component?

Question

Node.js

API node:

var express = require('express');
var router = express.Router();

var ch = 0
router.post('/', function(req, res) {
  ch += 1
  res.json({
    ch
  })
})

router.get('/c', function(req, res, next) {
  res.json(
    ch
  )
})

module.exports = router;

react component:

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

class App extends Component {
    state = {
        count: []
    }

    componentDidMount() {
        fetch('/users/c')
            .then(res => res.json())
            .then(count => this.setState({ count }));
    }

    render() {
        return (
            <div>
                {this.state.count}
                <button onClick={this.add}>add</button>
            </div>
        )
    }
    add = () => {
        var xhr = new XMLHttpRequest();
        xhr.open('POST', '/users/');
        xhr.send();
    }
}

export default App;

I receive a zero from the server and display. then when I click on the `add` button, I pull the POST request to add one. One is added, but what would be displayed, you need to refresh the page. Probably it is necessary to do something with setState. but I don't understand what

Answer 1

add = () => {
  fetch('/users/', { method: 'POST' })
    .then(res => res.json())
    .then(count => this.setState({ count }));
};