Select2 4 custom data adapter

you define it via AMD-Pattern:

$.fn.select2.amd.define('select2/data/customAdapter',[
        'select2/data/array',
        'select2/utils'
    ],
    function (ArrayAdapter, Utils) {

        function CustomDataAdapter ($element, options) {
            CustomDataAdapter.__super__.constructor.call(this, $element, options);
        }
        Utils.Extend(CustomDataAdapter, ArrayAdapter);

        CustomDataAdapter.prototype.current = function (callback) {

            callback(...);

        };

        return CustomDataAdapter;
    }
);

var customAdapter=$.fn.select2.amd.require('select2/data/customAdapter');

$("#my").select2({
    tags: true, 
    dataAdapter: customAdapter
});

For anyone trying to extend select2, here is an example :

// Require the adapter you want to override
$.fn.select2.amd.require(["select2/data/select"], function (Select) {
    let CustomDataAdapter = Select;

    // Simple example, just override the function
    CustomDataAdapter.prototype.current = function (callback) {
        // Your own code
    };

    // Example modifying data then calling the original function (which we need to keep)
    let originalSelect = CustomDataAdapter.prototype.select;
    CustomDataAdapter.prototype.select = function (data) {
        // Your own code
        // Call the original function while keeping 'this' context
        originalSelect.bind(this)(data);
    };

    // Finally, use the custom data adapter
    $('#my-select').select2({
        dataAdapter: CustomDataAdapter
    });

});