Answers for "select2 which value is selected"

select2 multi select get selected value

alert("Selected value is: "+$(".leaderMultiSelctdropdown").select2("val"));

alternatively, if you used a regular selectbox as base, you should be able to
use the normal jquery call, too:

alert("Selected value is: "+$(".leaderMultiSelctdropdown").val());

select 2 select a value

SELECT2 V4 :
============
For select2 v4 you can append directly an option/s as follow:

<select id="myMultipleSelect2" multiple="" name="myMultipleSelect2[]">
    <option value="TheID" selected="selected">The text</option>                                                                   
</select>

Or with JQuery:
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var $newOption = $("<option selected='selected'></option>").val("TheID")
					.text("The text")
 
$("#myMultipleSelect2").append($newOption).trigger('change');

other example :
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$("#myMultipleSelect2").val(5).trigger('change');