JSFiddle - React, Tailwind, and code Playground

HTML

<button type="button">Testar</button>

JavaScript

function testaLigacao(callback) {
    var xhr = new XMLHttpRequest();
    var url = "http://sergiofrilans.se";
    var novoNumero = new Date().getTime();

    xhr.open('HEAD', url + "?rand=" + novoNumero, true);
    xhr.onreadystatechange = function () {
        if (xhr.readyState != 4) return;
        if (xhr.status >= 200 && xhr.status < 304) return callback(true);
        return callback(false);
    }
    try {
        xhr.send();
    } catch (e) {
        return callback(false);
    }
}

document.querySelector('button').addEventListener('click', function () {
    testaLigacao(function (online) {
        alert('O browser está ' + (online ? 'online.' : 'offline.'));
    });
});