JSFiddle - React, Tailwind, and code Playground

HTML

<div>
</div>
<p></p>
<p></p>

CSS

div { display: none; }

JavaScript

var div = document.getElementsByTagName('div')[0];
for(var i=0; i<2000; ++i) {
    var inpt = document.createElement('input');
    inpt.type = 'checkbox';
    inpt.checked = true;
    var stringified = String(i);
    var n = ['xxxxxxxx'];
    for (var j=0; j < (8 - stringified.length); ++j) {
        n.push('0');
    }
    n.push(stringified);
    inpt.name = n.join('');
    div.appendChild(inpt);
}

var tabDemande = "";
var inputs = document.getElementsByTagName("input");

for (var i=0; i<inputs.length; i++){
    var input = inputs[i];
    if (input.type !== 'checkbox' || !input.checked) {
        continue;
    }

    var demande_id = input.name.substring(8);
    tabDemande += demande_id + '/';
}
document.getElementsByTagName('p')[0].textContent = tabDemande;
document.getElementsByTagName('p')[1].textContent = tabDemande.length;