Simple cross-browser AJAX method

ajax.js

// Define our own ajax method which uses XMLHttpRequest to fetch data from our Node.js app and OMDBapi.com
function ajax(url, method, callback, data, header) {
    try {
        var req = new(XMLHttpRequest || ActiveXObject)('MSXML2.XMLHTTP.3.0'); // Does XMLHttpRequest exist? If not, it's probably an older version of Internet Explorer
        req.open(method, url, true); // the method is either GET or POST in our case, the url which we want to call to get data from and TRUE to make it an asynchronous request
        if (header) {
            req.setRequestHeader("Content-type", header); // Set a custom header
        }
        req.onreadystatechange = function () {
            req.readyState > 3 && callback && callback(req.responseText, req); // When the response comes back call the callback function
        };

        if (data) {
            req.send(data) // Send with data
        } else {
            req.send(); // Send without data
        }

    } catch (e) {
        window.console && console.error(e); // If there is an error, show it in the browser's F12 console
    }
};