Like this (source: here and here):
var getUrlParameter = function getUrlParameter(sParam) {
    var sPageURL = decodeURIComponent(window.location.search.substring(1)),
        sURLVariables = sPageURL.split('&'),
        sParameterName,
        i;
    for (i = 0; i < sURLVariables.length; i++) {
        sParameterName = sURLVariables[i].split('=');
        if (sParameterName[0] === sParam) {
            return sParameterName[1] === undefined ? true : sParameterName[1];
        }
    }
};
Usage example:
var x = getUrlParameter('x');
var y = getUrlParameter('y');
EDIT: Or if you also use PHP for this site you could just do:
var x = <?php echo json_encode($_GET['x']); ?>