JSFiddle - React, Tailwind, and code Playground

HTML

<div class="myclass" data-n="group1"></div>
<div class="myclass" data-n="group1"></div>
<div class="myclass" data-n="group1"></div>
<div class="myclass" data-n="group2"></div>
<div class="myclass" data-n="group2"></div>
<div class="myclass" data-n="group3"></div>

JavaScript

$(document).ready(function() {

    var group = {};
    
    $('.myclass').each(function(){
        
        if (group[$(this).attr('data-n')] != true) {
            group[$(this).attr('data-n')] = true;
            
            //do something here once per each group
            alert('Group: '+ $(this).attr('data-n'));
        }
    
    });
 
});