JSFiddle - React, Tailwind, and code Playground

HTML

<div class="myClass">1</div>
<div class="myClass">2</div>
<div class="myClass">3</div>
<div class="myClass">4</div>
<div class="myClass">5</div>

JavaScript

$(".myClass").each(function(i){
    if( i % 2 == 0 ){
        $(this).data("myAttr",i + 1);                
    }
});
    
var res = $(".myClass").map(function(i) {
    console.log($(this).data("myAttr"));
    return $.data(this, "myAttr") !== undefined ? this : null
});    

console.log(res);