var chap_array = new Array(
    ["20",'20'],

    ["19",'19'],

    ["18",'18'],

    ["17",'17'],

    ["16",'16'],

    ["15",'15'],

    ["14",'14'],

    ["13",'13'],

    ["12",'12'],

    ["11",'11'],

    ["10",'10'],

    ["9",'9'],

    ["8",'8'],

    ["7",'7'],

    ["6",'6'],

    ["5",'5'],

    ["4",'4'],

    ["3",'3'],

    ["2",'2'],

    ["1",'1']
);
for (var i=0; i<chap_array.length; i++) {
    document.getElementById('id_chapter_1').options[i] = new Option(chap_array[i][0],chap_array[i][1]);
    if (document.getElementById('id_chapter_2')) {
        document.getElementById('id_chapter_2').options[i] = new Option(chap_array[i][0],chap_array[i][1]);
    }
}