关于javascrip的立即执行函数的解析

var xmlhttpUtil = function () {    
function getText(url,callback){
        alert("dog");
}();

(function(b){
            alert(b);
        })(321);

var xmlhttpUtil = function () {
    function getText(url,callback){
        var request=new XMLHttpRequest();
        request.open("GET",url);
        request.send(null);
        request.onreadystatechange=function(){
            if(request.readyState===4 && request.status===200){
                var type=request.getResponseHeader("Content-Type");
                if(type.match(/^text/))
                    callback(request.responseText);
                    alert(request.responseText);
            }
            else {

            }
        }
    }
    let xmlHttpRequestObj={};
    xmlHttpRequestObj.getText=getText;
    return xmlHttpRequestObj;
}();

xmlhttpUtil.getText("/dog",function () {
    alert("xmlhttprequest请求响应！");
});