Simple Ajax only for JSONP

myAjax.js

/*! Simple Ajax only for JSONP
 *
 * var args = {
 *     key: 'my-key',
 *     format: 'json'
 * };
 * myAjax.get('http://example.com/', args,
 *     function (response) {
 *         for (var key in response) {
 *             // key, response[key]
 *         }
 *     },
 *     function (err) {
 *         // err = {}
 *     }
 * );
 */
myAjax = {
    handler: [],
    response: [],
    serialize: function (base, args) {
        var query = [];
        for (var key in args) {
            query.push(
                encodeURIComponent(key) + '=' +
                encodeURIComponent(args[key])
            );
        }
        return base + '?' + query.join('&');
    },
    get: function (url, callback) {
        var index = myAjax.handler.length;
        var handler = 'myAjax.handler[' + index + ']';

        // Regist response handler for each request
        myAjax.handler.push(function (response) {
            myAjax.response[index] = response;
        });

        var i = 1, args = {}, onerror;
        if (typeof callback !== 'function') {
            args = arguments[i++];
            callback = arguments[i];
        }
        args.callback = handler;
        onerror = arguments[++i];

        var req = document.createElement('script');

        // Regist user callback function
        if (callback) {
            req.onload = req.onreadystatechange = function () {
                if (!this.readyState ||
                    this.readyState === "loaded" ||
                    this.readyState === "complete") {
                    this.onreadystatechange = null;
                    callback(myAjax.response[index]);
                }
            };
        }

        if (onerror) {
            req.onerror = onerror;
        }

        req.async = true;
        req.type = 'text/javascript';
        req.src = myAjax.serialize(url, args);
        document.body.appendChild(req);
    }
};