用原生JavaScript实现jQuery的$.getJSON

var $ = {
    getJSON: function(url, params, callbackFuncName, callback){
        var paramsUrl ="",
            jsonp = this.getQueryString(url)[callbackFuncName];
        for(var key in params){
            paramsUrl+="&"+key+"="+encodeURIComponent(params[key]);
        }
        url+=paramsUrl;
        window[jsonp] = function(data) {
            window[jsonp] = undefined;
            try {
                delete window[jsonp];
            } catch(e) {}

            if (head) {
                head.removeChild(script);
            }
            callback(data);
        };

        var head = document.getElementsByTagName('head')[0];
        var script = document.createElement('script');
        script.charset = "UTF-8";
        script.src = url;
        head.appendChild(script);
        return true;
    },
    getQueryString: function(url) {
        var result = {}, queryString = (url && url.indexOf("?")!=-1 && url.split("?")[1]) || location.search.substring(1),
            re = /([^&=]+)=([^&]*)/g, m;
        while (m = re.exec(queryString)) {
            result[decodeURIComponent(m[1])] = decodeURIComponent(m[2]);
        }
        return result;
    }
};

var url = "http://xxx.xxx.xxx?callback=jsonp123";
var params = {
    a:1,
    b:2
};
$.getJSON(url, params, "callback", function(data){
    //todo
});