I use this pseudo-class to make ajax request to server:

function RequestManager(url, params, success, error){
  //save this ajax configuration
  this._ajaxCall = null;
  this._url= url;
  this._type = params.type;
  this._timer = params.timer || 0;
  this._success = function(){
    alert("ok");
  };
  this._error = function(){
    alert("ko");
  };
}

RequestManager.prototype = { 
  require : function(tag){//here there will be a tag object in array format
    if(this.timer>0){
        //wait this.timer
    }

    if(this.ajaxCall != null ){
        this.ajaxCall.abort();
        this.ajaxCall = null;
    }

    var self = this;
    this.ajaxCall = $.ajax({
        url: this._url,
        type: this._type,
        data: tag,
        success: function(xmlResponse){
            var responseArray = [];
            var response = _extractElements(xmlResponse, arrayResponse);
            self._success(response);
        },
        error: self._error,
        complete : function(xhr, statusText){
            alert(xhr.status);
            return null;
        }
  });
}

When i instantiate a new object, and i use it to load a request, it always return to me error, and status code is 0. Server correctly generates an xml document. Why i can't get a correct 200 code back?