var windowHeight = document.documentElement.clientHeight;
var popupVisible = false;

function showPopup(show) {
    if(show && !popupVisible) {
        $("#popup").fadeIn("fast");
        $("#popup-background").fadeIn("fast");
        popupVisible = true;
    }
    else if(!show && popupVisible) {
        $("#popup-background").fadeOut("fast");
        $("#popup").fadeOut("fast");
        popupVisible = false;
    }
}

$(document).ready(function() {
    $("#popup").css("left", (document.documentElement.clientWidth / 2 - $("#popup").width() / 2) + "px");
    $("#popup").css("top", (windowHeight / 2 - $("#popup").height() / 2) + "px");

    $("#popup-background").css("height", windowHeight);
    $("#popup-background").css("opacity", "0.85");

    $(".tell-a-friend").click(function() {
        showPopup(true);
    });

    $("#popup .close").click(function() {
        showPopup(false);
    });
});