javascript内部的Ajax调用

var oldip = document.getElementById('ip').value;
var newip1 = document.getElementById('txtintip1').value;
var newip2 = document.getElementById('txtintip2').value;
var newip3 = document.getElementById('txtintip3').value;
var newip4 = document.getElementById('txtintip4').value;
var newip = newip1 + "." + newip2 + "." + newip3 + "." + newip4;
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function () {
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
          if (xmlhttp.responseText == "pinging") {
              alert("IP is already in use");
          }         
    }
}
xmlhttp.open("GET", "checkping.php?ip=" +newip, true);
xmlhttp.send();
if (document.getElementById('interface').value == "default") {
    alert("Select Interface");
    document.registerlist.interface.focus();
    return false;
} 
if (document.getElementById('txtintip1').value === "") {
    alert("Enter Valid IP Address");
    document.registerlist.txtintip1.focus();
    return false;
}
if (document.getElementById('txtintip2').value === "") {
    alert("Enter Valid IP Address");
    document.registerlist.txtintip2.focus();
    return false;
}
if (document.getElementById('txtintip3').value === "") {
    alert("Enter Valid IP Address");
    document.registerlist.txtintip3.focus();
    return false;
}
if (document.getElementById('txtintip4').value === "") {
    alert("Enter Valid IP Address");
    document.registerlist.txtintip4.focus();
    return false;
}

以上是我的代码，其中第一个ajax将检索是否给定的ip是冲突的。但我面临的问题是，ajax是响应缓慢，javascript是完整的。所以我不能完成我的动作，所以如何先运行ajax，然后javascript执行将继续?