getJSON AJAX response, IE11 compat

getJSON.js

/**
    * Make AJAX call for JSON resource
    * Supports IE11 and browsers that do not allow responseType='json'.
    * https://mathiasbynens.be/notes/xhr-responsetype-json
    * ----------------------------------------------------- */
var getJSON = function(url, successHandler, errorHandler) {
    var xhr = typeof XMLHttpRequest != 'undefined' ? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP');

    var supportsJSON = (function() {
        if (typeof XMLHttpRequest == 'undefined') { return false; }
        var xhr = new XMLHttpRequest();
        xhr.open('get', '/', true);
        try {
            // some browsers throw when setting `responseType` to an unsupported value
            xhr.responseType = 'json';
        } catch(error) { return false; }
        return 'response' in xhr && xhr.responseType == 'json';
    }());

    xhr.open('get', url, true);
    if (supportsJSON) { xhr.responseType = 'json'; }
    xhr.onreadystatechange = function() {
        var status = xhr.status;
        var data;
        // https://xhr.spec.whatwg.org/#dom-xmlhttprequest-readystate
        if (xhr.readyState == 4) { // `DONE`
            if (status == 200) {
                successHandler && successHandler( supportsJSON ? xhr.response : JSON.parse(xhr.responseText));
            } else {
                errorHandler && errorHandler(status);
            }
        }
    };
    xhr.send();
}