<select class="form-control form-small">
    <option selected="selected">orange</option>
    <option>white</option>
    <option>purple</option>
</select>

var formSmall = $(".form-small").select2({ tags: true });
formSmall.data('select2').$container.addClass('form-control-sm')

<select class="form-control nested">
    <optgroup label="Group1">
        <option selected="selected">orange</option>
        <option>white</option>
        <option>purple</option>
    </optgroup>
    <optgroup label="Group2">
        <option>purple</option>
        <option>orange</option>
        <option>white</option>
    </optgroup>
    <optgroup label="Group3">
        <option>white</option>
        <option>purple</option>
        <option>orange</option>
    </optgroup>
</select>

$(".nested").select2({
    tags: true
});

<select class="form-control disabled-results">
    <option value="one">First</option>
    <option value="two" disabled="disabled">Second</option>
    <option value="three">Third</option>
</select>

$(".disabled-results").select2();

<select class="placeholder js-states form-control">
    <option>Choose...</option>
    <option value="one">First</option>
    <option value="two">Second</option>
    <option value="three">Third</option>
    <option value="four">Fourth</option>
    <option value="five">Fifth</option>
</select>

$(".placeholder").select2({
    placeholder: "Make a Selection",
    allowClear: true
});

<select class="form-control tagging" multiple="multiple">
    <option>orange</option>
    <option>white</option>
    <option>purple</option>
</select>

$(".tagging").select2({
    tags: true
});

<select class="form-control tagging" multiple="multiple">
    <option>orange</option>
    <option>white</option>
    <option>purple</option>
</select>

var ss = $(".basic").select2({
    tags: true,
});