var request = null;
try {
  request = new XMLHttpRequest();
} catch (essaimicrosoft) {
  try {
    request = new ActiveXObject("Msxml2.XMLHTTP");
  } catch (autremicrosoft) {
    try {
      request = new ActiveXObject("Microsoft.XMLHTTP");
    } catch (echec) {
      request = null;
    }
  }
}

if (request == null)
  alert("Impossible de créer l'objet requête!");