AJAX advanced

ajax.js

function ajax(o) {
    'use strict';

    var xhr = new XMLHttpRequest();
    if (o.withCredentials) { xhr.withCredentials = true; }
    xhr.open(o.method || 'GET', o.url, true);
    var cbInner = function() {
        if (xhr.readyState === 4 && xhr.status > 199 && xhr.status < 300) {
            return o.cb(null, JSON.parse(xhr.response));
        }
        o.cb('error requesting ' + o.url);
    };
    xhr.onload  = cbInner;
    xhr.onerror = cbInner;
    xhr.send(o.body || null);
}