Basic usage

<select class="form-select" id="basic-usage" data-placeholder="Choose one thing">
    <option></option>
    <option>Reactive</option>
    <option>Solution</option>
    <option>Conglomeration</option>
    <option>Algoritm</option>
    <option>Holistic</option>
</select>

$( '#basic-usage' ).select2( {
    theme: "bootstrap-5",
    width: $( this ).data( 'width' ) ? $( this ).data( 'width' ) : $( this ).hasClass( 'w-100' ) ? '100%' : 'style',
    placeholder: $( this ).data( 'placeholder' ),
} );

Select2 versions

There are 2 versions of Select2 which changed some options. The Select2 Bootstrap 5 theme supports both 4.0.x and 4.1.x-rc.x. Which version of Select2 you use will change some of the usage for the Select2 Bootstrap 5 theme.

Sizing

There are 2 ways to set the sizing of the Select2 element; using Select2 options, or using Bootstrap 5 classes and Select2 options. Between 4.0.x and 4.1.x-rc.x, Select2 changed the option name for adding classes to the main element (the one you see before opening the dropdown) from containerCssClass to selectionCssClass. See below for an example

// For Select2 4.0
$("#form-select-sm").select2({
    theme: "bootstrap-5",
    containerCssClass: "select2--small",
    dropdownCssClass: "select2--small",
});

// For Select2 4.1
$("#form-select-sm").select2({
    theme: "bootstrap-5",
    selectionCssClass: "select2--small",
    dropdownCssClass: "select2--small",
});