<!DOCTYPE html>
<html>
  <head>
    <!-- bootstrap -->
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
    
    <!-- select2 -->
    <link href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.6-rc.0/css/select2.min.css" rel="stylesheet" />
    
    <!-- select2-bootstrap4-theme -->
    <link href="select2-bootstrap4.min.css" rel="stylesheet">
  </head>

  <body>
    
    <div class="container mt-5">
      <h3>Example of <a href="https://github.com/ttskch/select2-bootstrap4-theme" target="_blank">@ttskch/select2-bootstrap4-theme</a></h3>
      <hr>
      
      <form>
        <div class="form-group">
          <label>Example select</label>
          <select class="form-control" id="select-single" placeholder="Choose one thing" data-allow_clear="1">
            <option></option>
            <option>1</option>
            <option>2</option>
            <option>3</option>
            <option>4</option>
            <option>5</option>
          </select>
        </div>
        <div class="form-group">
          <label>Example multiple select</label>
          <select multiple class="form-control" placeholder="Choose anything" data-allow_clear="1">
            <option>1</option>
            <option>2</option>
            <option>3</option>
            <option>4</option>
            <option>5</option>
          </select>
        </div>
      </form>
    </div>

    <!-- bootstrap -->
    <script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js" integrity="sha384-ApNbgh9B+Y1QKtv3Rn7W3mgPxhU9K/ScQsAP7hUibX39j7fakFPskvXusvfa0b4Q" crossorigin="anonymous"></script>
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl" crossorigin="anonymous"></script>
    
    <!-- select2 -->
    <script src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.6-rc.0/js/select2.min.js"></script>
    
    <!-- select2-bootstrap4-theme -->
    <script src="script.js"></script>
  </body>
</html>
$(function () {
  $('select').each(function () {
    $(this).select2({
      theme: 'bootstrap4',
      placeholder: $(this).attr('placeholder'),
      allowClear: Boolean($(this).data('allow_clear')),
    });
  });
});
.select2-container--bootstrap4 .select2-selection--single{height:calc(2.25rem + 2px)!important}.select2-container--bootstrap4 .select2-selection--single .select2-selection__placeholder{color:#757575;line-height:2.25rem}.select2-container--bootstrap4 .select2-selection--single .select2-selection__arrow{position:absolute;top:50%;right:3px;width:20px}.select2-container--bootstrap4 .select2-selection--single .select2-selection__arrow b{top:60%;border-color:#343a40 transparent transparent;border-style:solid;border-width:5px 4px 0;width:0;height:0;left:50%;margin-left:-4px;margin-top:-2px;position:absolute}.select2-container--bootstrap4 .select2-selection--single .select2-selection__rendered{line-height:2.25rem}.select2-search--dropdown .select2-search__field{border:1px solid #ced4da;border-radius:.25rem}.select2-results__message{color:#6c757d}.select2-container--bootstrap4 .select2-selection--multiple{min-height:calc(2.25rem + 2px)!important}.select2-container--bootstrap4 .select2-selection--multiple .select2-selection__rendered{-webkit-box-sizing:border-box;box-sizing:border-box;list-style:none;margin:0;padding:0 5px;width:100%}.select2-container--bootstrap4 .select2-selection--multiple .select2-selection__choice{color:#343a40;border:1px solid #bdc6d0;border-radius:.2rem;padding:0;padding-right:5px;cursor:pointer;float:left;margin-top:.3em;margin-right:5px}.select2-container--bootstrap4 .select2-selection--multiple .select2-selection__choice__remove{color:#bdc6d0;font-weight:700;margin-left:3px;margin-right:1px;padding-right:3px;padding-left:3px;float:left}.select2-container--bootstrap4 .select2-selection--multiple .select2-selection__choice__remove:hover{color:#343a40}.select2-container :focus{outline:0}.select2-container--bootstrap4 .select2-selection{border:1px solid #ced4da;border-radius:.25rem;width:100%}.select2-container--bootstrap4.select2-container--focus .select2-selection{border-color:#17a2b8;-webkit-box-shadow:0 0 0 .2rem rgba(0,123,255,.25);box-shadow:0 0 0 .2rem rgba(0,123,255,.25)}.select2-container--bootstrap4.select2-container--focus.select2-container--open .select2-selection{border-bottom:none;border-bottom-left-radius:0;border-bottom-right-radius:0}select.is-invalid~.select2-container--bootstrap4 .select2-selection{border-color:#dc3545}select.is-valid~.select2-container--bootstrap4 .select2-selection{border-color:#28a745}.select2-container--bootstrap4 .select2-dropdown{border-color:#ced4da;border-top:none;border-top-left-radius:0;border-top-right-radius:0}.select2-container--bootstrap4 .select2-dropdown .select2-results__option[aria-selected=true]{background-color:#e9ecef}.select2-container--bootstrap4 .select2-results__option--highlighted,.select2-container--bootstrap4 .select2-results__option--highlighted.select2-results__option[aria-selected=true]{background-color:#007bff;color:#f8f9fa}.select2-container--bootstrap4 .select2-results__option[role=group]{padding:0}.select2-container--bootstrap4 .select2-results__group{padding:6px;display:list-item;color:#6c757d}.select2-container--bootstrap4 .select2-selection__clear{width:1.2em;height:1.2em;line-height:1.15em;padding-left:.3em;margin-top:.5em;border-radius:100%;background-color:#6c757d;color:#f8f9fa;float:right;margin-right:.3em}.select2-container--bootstrap4 .select2-selection__clear:hover{background-color:#343a40}