Select2 demo

HTML

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.2.1/css/bootstrap.min.css">
<link rel="stylesheet" href="https://rawgit.com/select2/select2/master/dist/css/select2.min.css">
<script src="https://code.jquery.com/jquery-3.3.1.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.6/umd/popper.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.2.1/js/bootstrap.min.js"></script>
<script src="https://rawgit.com/select2/select2/master/dist/js/select2.js"></script>
<form class="was-validated">

  <div class="form-group">
    <select class="custom-select" required>
            <option value="">Open this select menu</option>
            <option value="1">One</option>
            <option value="2">Two</option>
            <option value="3">Three</option>
        </select>
    <div class="invalid-feedback">Example invalid custom select feedback</div>
  </div>

  <div class="form-group">
    <select class="select custom-select" required>
            <option value="">Open this select menu</option>
            <option value="1">One</option>
            <option value="2">Two</option>
            <option value="3">Three</option>
        </select>
    <div class="invalid-feedback">Example invalid custom select feedback</div>
  </div>

</form>

JavaScript

$(".select").select2({
  minimumResultsForSearch: Infinity
});