JSFiddle - React, Tailwind, and code Playground
HTML
<button onClick="addCommas">click here</button>
JavaScript
function addCommas()
{
nStr += '98765432154698745123986451';
x = nStr.split('.');
x1 = x[0];
x2 = x.length > 1 ? '.' + x[1] : '';
var rgx = /(\d+)(\d{3})/;
while (rgx.test(x1)) {
x1 = x1.replace(rgx, '$1' + ',' + '$2');
}
return x1 + x2;
}