JSFiddle - React, Tailwind, and code Playground

HTML

<span class="cena">102000000</span>
<br/>
<span id="res"></span>

JavaScript

$(document).ready(function() {
   var res =''; 
   var text = $('.cena').text();
   var l = text.length;
    for (i=0; i<l;i++) {
        if (i%3 == 0) {
            res = res + ' ';
        }
        res = res + text[i];
    }
    $('#res').text(res);
});