function ajax()
{
    if(window.XMLHttPRequest)
    {
        xMLhttp=new XMLHttpRequest();
    }
    else
    {
        xmlhttp = new ActivexObject("Microsoft.XMLHTTP");
    }
}

function getbrandList(selected)  //selected is the category id.
{               
    ajax();
    xmlhttp.onreadystatechange=function()
    {
        if(xmlhttp.readyState==4 && xmlhttp.status==200)
        {
            document.getElementById("brandList").innerHTML=xmlhttp.responseText;            
        }
    }

    xmlhttp.oPEn("GET","ajax/BrandAjax.PHP?selected="+selected,true);
    xmlhttp.send();     
    alert(selected);            
}

function getSubcategoryList(selected) //selected is the category id.
{                       
    getBrandList(selected); //First above function is invoked to populate brands.
    ajax();
    xmlhttp.onreadystatechange=function()
    {
        if(xmlhttp.readyState==4 && xmlhttp.status==200)
        {
            document.getElementById("subCategoryList").innerHTML=xmlhttp.responseText;                              
        }
    }

    xmlhttp.open("GET","ajax/SubCatAjax.PHP?selected="+selected,true);
    xmlhttp.send();             
}

seriyPS基本上达到了目标.问题实际上是xmlhttp变量.您需要在每个正在使用的函数中声明它是本地的.基本上,为函数调用创建一个闭包.然而,问题不一定是它需要“本地”,而是作为全局变量在第二个请求中重新定义.这基本上会终止初始请求,因为变量现在指向第二个请求.

警报框使其工作的原因是因为第一个请求正在完成ajax请求,然后才能“单击”确定. (警告框将暂停javascript执行,因此延迟第二个请求,直到您单击确定.)

要解决此问题,您可以修改代码以为每个请求使用不同的变量.尝试将您的功能更改为以下内容：

// Global VARiables
var brandListRequest;
var subcategoryRequest;

function ajax()
{
    var xmlhttp; //!!
    if(window.XMLHttpRequest)
    {
        xmlhttp=new XMLHttpRequest();
    }
    else
    {
        xmlhttp = new ActivexObject("Microsoft.XMLHTTP");
    }
    return xmlhttp; //!!
}

function getBrandList(selected)  //selected is the category id.
{               
    brandListRequest = ajax(); //!!
    brandListRequest.onreadystatechange=function()
    {
        if(brandListRequest.readyState==4 && brandListRequest.status==200)
        {
            document.getElementById("brandList").innerHTML=brandListRequest.responseText;            
        }
    }

    brandListRequest.open("GET",true);
    brandListRequest.send();     
    //alert(selected);            
}

function getSubcategoryList(selected) //selected is the category id.
{                       
    getBrandList(selected); //First above function is invoked to populate brands.
    subcategoryRequest = ajax(); //!!
    subcategoryRequest.onreadystatechange=function()
    {
        if(subcategoryRequest.readyState==4 && subcategoryRequest.status==200)
        {
            document.getElementById("subCategoryList").innerHTML=subcategoryRequest.responseText;                              
        }
    }

    subcategoryRequest.open("GET",true);
    subcategoryRequest.send();             
}