var xhr = null;
function constructXhr() {
	if(window.XMLHttpRequest) // Firefox et autres
		xhr = new XMLHttpRequest(); 
	else if(window.ActiveXObject) { // Internet Explorer 
		try {
			xhr = new ActiveXObject("Msxml2.XMLHTTP");
		} catch (e) {
			xhr = new ActiveXObject("Microsoft.XMLHTTP");
		}
	}
	else { // XMLHttpRequest non supporté par le navigateur 
		alert("Your browser does not support XMLHTTPRequest...");
	}
}