@ttskch/select2-bootstrap4-theme/docs/script.js

Select2 v4 theme for Bootstrap4

$(function () {
  $('select').each(function () {
    $(this).select2({
      theme: 'bootstrap4',
      width: $(this).data('width') ? $(this).data('width') : $(this).hasClass('w-100') ? '100%' : 'style',
      placeholder: $(this).data('placeholder'),
      allowClear: Boolean($(this).data('allow-clear')),
      closeOnSelect: !$(this).attr('multiple'),
    });
  });
});