var modal_scrollTop = $('.modal-body').scrollTop();
var modal_scrollHeight = $('.modal-body').prop('scrollHeight');
var modal_innerHeight = $('.modal-body').innerHeight();

$('.modal-body').scroll(function() {

    // Write to console log to debug:
    console.warn('modal_scrollTop: ' + modal_scrollTop);
    console.warn('modal_innerHeight: ' + modal_innerHeight);
    console.warn('modal_scrollHeight: ' + modal_scrollHeight);

    // Bottom reached:
    if (modal_scrollTop + modal_innerHeight >= (modal_scrollHeight - 100)) {
        alert('reached bottom');
    } 

});