var xhr_object = null;

function create_xhr_object() {
	if (window.XMLHttpRequest) // Firefox
	  return new XMLHttpRequest();
	else if (window.ActiveXObject) // Internet Explorer
	  return new ActiveXObject("Microsoft.XMLHTTP");
	else // Navigateur non supporté
	  alert("navigateur non supporte.");
}

xhr_object = create_xhr_object();