Ajax renvoi true ou false

function demande(pseudo)
{		
    var xhr = getXMLHttpRequest();
    xhr.onreadystatechange = function()
    {
        if (xhr.readyState == 4 && (xhr.status == 200 || xhr.status == 0))
        {
	if(xhr.responseText == "libre")
	{
	    alert(pseudo.value+" est libre");
	    return true;
	}
	else
            {
	    alert(pseudo.value+" déja pris !");
	    return false;
            }
        }
    }
    xhr.open("POST", "php/ajax.verif_pseudo.php", true);
    xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    xhr.send("pseudo="+pseudo.value);
}