function _createXHR () {
  return XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP');
}

var xhr = _createXHR();
var url = "Default.aspx?username=erichow&password=123";

xhr.onreadystatechange = function() {
  if (xhr.readyState == 4 && xhr.status == 200) {
    console.log(xhr.responseText);
  }
};

//get 方式
xhr.open('get', url, true);
xhr.send(null);

//post 方式
xhr.open('post', url, true);
xhr.setRequestHeader('Content-Type','application/x-www-form-urlencoded');  
xhr.send('username=erichow&password=123);