XMLHttpRequest with Async Ajax Call

var client = new XMLHttpRequest();
var URL='/Home/Index';
client.open('POST',URL, true);  // parameters  for Open (method:string , url:string , async?:boolean , user?:string,password?:string)
client.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
client.setRequestHeader("dataType", "html");
client.send("para1=" + parmetervalue1 + "¶2=" + parmetervalue2); 
client.onloadend = function (response) {
    return response;
} 
client.error = function (ex) {
   window.location.reload();
}