Get all data-* attribute values of select options in jQuery

I have a chained dropdown menus witch are updated through AJAX request. I made a custom attribute to hold country Id for every province option. then I call on change function to call another function to update province options list based on country selection. however, every time I tried to get country ID for every province to filter them I got undefined. in this line of code : var data = $("#optProvince option").data('chained'); The debug for frontend, there is nothing wrong for backend I want some one to debug my code NOT to change it. The simple should not take more than 20 minutes for professional at the most.