[danik.js]

var request = new XMLHttpRequest();
request.open('GET', 'b.html');
request.onreadystatechange = function() {
    if (request.readyState == 4) {
        if (request.status == 200) {
            currentMenuItem.innerHTML = request.responseText;
        } else {
            alert('Сетевая ошибка, код: ' + request.status);
        }
    }
};
request.send(null);