Submit a form with Ajax

const submit = function (formEle) {
    return new Promise(function (resolve, reject) {
        // Serialize form data
        // See https://phuoc.ng/html-dom/serialize-form-data-into-a-query-string
        const params = serialize(formEle);

        // Create new Ajax request
        const req = new XMLHttpRequest();
        req.open('POST', formEle.action, true);
        req.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');

        // Handle the events
        req.onload = function () {
            if (req.status >= 200 && req.status < 400) {
                resolve(req.responseText);
            }
        };
        req.onerror = function () {
            reject();
        };

        // Send it
        req.send(params);
    });
};

const formEle = document.getElementById(...);

submit(formEle).then(function(response) {
    // `response` is what we got from the back-end
    // We can parse it if the server returns a JSON
    const data = JSON.parse(response);
    ...
});