js button click

//page

        var domain          = "http://localhost/new/";;
        var home            = "#home";
        var activities      = "#activities";
        var about           = "#about";
        var information     = "#information";
        var contact         = "#contact";

        var up          = document.getElementById("up");
        var dn          = document.getElementById("dn");

                                    //home page

        up.onclick = function()
        {
            if(document.URL == domain + home || document.URL == domain) { }
            else if(document.URL == domain + activities ) { window.location = domain + about; alert(); }
            else if(document.URL == domain + about )      { window.location = domain + information; alert(); }
            else if(document.URL == domain + information ) { window.location = domain + contact; alert(); }
            else if(document.URL == domain + contact )    { window.location = domain + about; alert(); }
            else {}
        }

        dn.onclick = function()
        {
            if(document.URL      == domain + home )       { window.location = domain + activities; alert(); }
            else if(document.URL == domain + activities )     { window.location = domain + about; alert(); }
            else if(document.URL == domain + about )      { window.location = domain + information; alert(); }
            else if(document.URL == domain + information ) { window.location = domain + contact;alert(); }
            else if(document.URL == domain + contact ) { window.location = domain + contact; alert(); }
            else {}
        }