JSFiddle - React, Tailwind, and code Playground

HTML

<span class="text"></span><br/>
<div class="allboxes">
  <input type="checkbox" value="a" name="idCheck">
  <input type="checkbox" value="b" name="idCheck">
  <input type="checkbox" value="c" name="idCheck">
  <input type="checkbox" value="d" name="idCheck">
</div>

JavaScript

$('.allboxes input').change(function() {
    var whichSelected = [];
    $.each($('.allboxes input:checked'), function(a,b) {
        whichSelected.push($(b).val());
        if(whichSelected != null){
        alert("ok");
        }else{
        alert("ko");
        }
        
    });
    $('.text').text(whichSelected.join(' '));
});