Url replace

$(document).ready(function() {
    var url = location.href;
    var bourn = url.lastIndexOf('/') + 1;
    var page = eval(url.substr(bourn));
    var extant =  url.substring(0,bourn);
    var next;
    $(document).keyup(function(e) {
        if (e.which == 39) {
            next = extant + (page+1);
            window.open(next, '_self');
        }
        if (e.which == 37) {
            next = extant + (page-1);
            window.open(next, '_self');
        }
    });
});