select2 dropdown: When loading results with ajax, load them once. (the results are always the same)

select2cacheResults.js

var s2data = $allUnitsSelect.select2({
            ajax: {
               url: '/GetValues',
                dataType: 'json',
                data: function (params) {
                    var query = {
                     text: text,
                 }

                  return query;
               },
               processResults: function (responseValues) {
                   var listItems = mapResponseData(responseValues);
                 
                   this.options.set('cacheDataSource', listItems);
                 
                  return { results: listItems };
               }
           },
            cacheDataSource: [],
            allowClear: true,
            placeholder: 'Select Values ...',
            width: '100%',
        }).data('select2');
        
s2data.dataAdapter.query = function (params, callback) {

    var cacheDataSource = this.options.get('cacheDataSource');
    if (cacheDataSource && cacheDataSource.length > 0) {
        callback({ results: cacheDataSource });
    }
    else {
        // call the original logic
        var ajaxAdapterFunc = jQuery.fn.select2.amd.require('select2/data/ajax');
        var ajaxAdapter = new ajaxAdapterFunc(this.$element, this.options);

        ajaxAdapter.query(params, callback);
    }
}

I do not know exactly your case (context) The will be the same every time or the data needs to be fetched each type when the user focuses out of the search field. The example above is complex for specified version of select and it will load the results with ajax only the first time. If the page is reloaded the select2 dropdown needs to make another ajax to load the data (It was very specific in my case).
If you are not using any library/framework like angular, backbone, react, vue.js etc. then localStorage, sessionStorage and cookies are available as option to store data.
Otherwise you can use Server side to render the data along with HTML. And then when initialize select2 use something like:

   <select id="dropdown-select2">
        <option value="val1">Text 1</option>
        <option value="val2">Text 2</option>
        <option value="val3">Text 3</option>
       ....
   </select>

  <script>
     $("#dropdown-select2").select2();
</script>

Or you can load the data from storage when the page loads:

(function() {
  var cacheData = [];  // this is accessible only inside that function, not globally

   $(document).ready(function() {
      var cacheData = JSON.parse(window.localStorage.getItem('data'));
    }) ;

    function initializeDropdown(element) {
        element.select2({
              data: cacheData
              // other options
        });
    } 
})()

But the property cacheDataSource in my example always starts with empty array and it is per dropdown.
In short, you cannot use that property when you want to initialize select2's data.

I had to update the code, else search stopped working

s2data.dataAdapter.query = function (params, callback) {
        var cacheDataSource = this.options.get('cacheDataSource');
        if (cacheDataSource && cacheDataSource.length > 0) {
            callback(
                {
                    results: cacheDataSource.filter(function (b) {return b.text && b.text.toLowerCase().includes(params.term.toLowerCase()); })
                });
        }
        else {
            var ajaxAdapterFunc = jQuery.fn.select2.amd.require('select2/data/ajax');
            var ajaxAdapter = new ajaxAdapterFunc(this.$element, this.options);
            ajaxAdapter.query(params, callback);
        }
    }

Hi @nj-git, nice catch.
After working with another dropdown libraries (in react) and hitting a lot of issues, be sure to handle the diacritics search. What I mean is when you have Français in the options and type Franc the search to return the options. I think this is enabled by default. Source