JSONP enable you to send request to other domain beyond the same domain policy. And the corss domain alternative to AJAX.

JSONP is natively supported by JQuery, Zepto (alternative to JQuery for mobile usages). But When you require JSONP and DO NOT want to add the huge JQuery to your code base. This library will be useful.

var JSONP = (function () {
    var counter = 0,
        head, query, key, window = this;

    function load(url) {
        var script = document.createElement('script'),
        var done = false;
        script.src = url;
        script.async = true;

        script.onload = script.onreadystatechange = function () {
            if (!done && (!this.readyState || this.readyState === "loaded" || this.readyState === "complete")) {
                done = true;
                script.onload = script.onreadystatechange = null;
                if (script && script.parentNode) {
                    script.parentNode.removeChild(script);
                }
            }
        };
        if (!head) {
            head = document.getElementsByTagName('head')[0];
        }
        head.appendChild(script);
    }

    function jsonp(url, params, error, callback) {
        query = "?";
        params = params || {};
        for (key in params) {
            if (params.hasOwnProperty(key)) {
                query += encodeURIComponent(key) + "=" + encodeURIComponent(params[key]) + "&";
            }
        }
        var jsonp = "json" + (++counter);
        window[jsonp] = function (data) {
            callback(data);
            try {
                delete window[jsonp];
            } catch (e) {}
            window[jsonp] = null;
        };

        load(url + query + "callback=" + jsonp);

        error = error ||
        function () {};

        window.setTimeout(function () {
            if (typeof window[jsonp] == "function") {

                // replace success with null callback in case the request is just very latent.
                window[jsonp] = function (data) {
                    try {
                        delete window[jsonp];
                    } catch (e) {}
                    window[jsonp] = null;
                };

                // call the error callback
                error();

                // set a longer timeout to safely clean up the unused callback.
                window.setTimeout(function () {
                    if (typeof window[jsonp] == "function") {
                        try {
                            delete window[jsonp];
                        } catch (e) {}
                        window[jsonp] = null;
                    };
                }, 120000);
            };
        }, 10000);

        return jsonp;
    }
    return {
        get: jsonp
    };
}());

/*
Example:
----------------

var url = 'http://blog.eood.cn/api';
var error = function() {alert('error');};
var success = function(data) {
        // process the data
};
JSONP.get( url, {'parm1': 'parm1_value', 'parm2': 'parm2_value'}, error, success);

*/