Back on the Node(ExpressJS) fetch API from WEB Browser

[RailBatyrshin]

Hi all, can someone explain to me, how does communicate frontend and backend? I have a project, that dividing into frontend and backend folders. And I can't understand how can I send the data from the registration form to the server-side using Fetch API. I want to create a SPA.
Thank you in advance, and sorry for my English.
Best regards!
P.s: The structure of my project in the attached files.
This is the code on the frontend:

const formData = {
                        name: name.value,
                        age: age.value.split('-').join('.'),
                        email: email.value,
                        password: pass.value,
                    };

                const checkResponseStatus = function(res) {
                    if (res.ok) {
                        return res;
                    } else {
                        throw new Error(`The HTTP status of the response: ${ res.status } (${ res.statusText })`);
                    }
                };

                const url = '/register';
                const request = new Request(url, {
                        method: 'POST',
                        body: JSON.stringify(formData),
                        headers: { 'Content-Type': 'application/json' }
                    });

                fetch(request)
                    .then(checkResponseStatus)
                    .then(response => response.json())
                    .catch(err => console.log(err));

                name.value = '';
                age.value = '';
                email.value = '';
                pass.value = '';
                repeatPass.value = '';

                for (let key of Object.keys(formData)) {
                    delete formData[key];
                }

And this is the code on the backend (server.js):

const express = require('express');
const bodyParser = require('body-parser');
const router = express.Router();

const app = express();

app.use(bodyParser.urlencoded({ extended: false }));
app.use(bodyParser.json());

app.use((req, res) => {
	const post_data = req.body;
	console.log(post_data);
});

router.get('/', (req, res) => {
	res.sendFile('../frontend/index.html');
});

router.post('/register', (req, res) => {
	const user_name = req.body.name;
	const password = req.body.password;
	console.log(`User name: ${ user_name }, password: ${ password }`);
	res.end('YES');
});

app.listen(3000, () => {
	console.log(`Started on PORT 3000`);
});