JSFiddle - React, Tailwind, and code Playground

HTML

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

JavaScript

function testaLigacao(callback) {
    var response;
    var callbackName = 'temp_callback_' + new Date().getTime() + Math.floor(Math.random() * 100);
    window[callbackName] = function (res) {
        delete window[callbackName];
        document.body.removeChild(script);
        response = res;
    };

    var src = 'http://teusite.com/ping.php?callback=' + callbackName;
    var script = document.createElement('script');
    script.src = src;
    script.onload = function () {
        callback(true);
    }
    script.onerror = function () {
        callback(false);
    }
    document.body.appendChild(script);
}

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