<a onclick="returnCategory($(this).attr('data-category-id'))" data-category-id="15">Tablets</a>

function returnCategory(categoryId) {
    return categoryId;
}

function showPopup() {
    $.fancybox({
        href: 'popup.php',
        type: 'ajax',
        width: '90%',
        height: '100%',
        beforeClose : function () {
            categoryId = returnCategory();
            alert("Required category Id is :" + categoryId);
        },
    });
}