var xhr;
if (window.XMLHttpRequest)
{// code for IE7+, Firefox, Chrome, Opera, Safari
    xhr=new XMLHttpRequest();
}
else
{// code for IE6, IE5
    xhr=new ActiveXObject("Microsoft.XMLHTTP");
}
// var xhr = new XMLHttpRequest();
xhr.open("POST","08-ajax-post.php",true);
// 注意点: 以下代码必须放到open和send之间
xhr.setRequestHeader("Content-type","application/x-www-form-urlencoded");
xhr.send("userName=zs&userPwd=321");
xhr.onreadystatechange = function (ev2) {
    if(xhr.readyState === 4){
        if(xhr.status >= 200 && xhr.status < 300 ||
        xhr.status === 304){
            // alert("请求成功");
            alert(xhr.responseText);
        }else{
            alert("请求失败");
        }
    }
}

ajax post请求