JSFiddle - React, Tailwind, and code Playground

JavaScript

var tests =
[
3141,
314159,
31415926,
10000000.001,
10333333.002,
42121.1415926535
],
res,
i;

function addCommas(nStr)
{
    nStr += '';
    var x = nStr.split('.');
    var x1 = x[0];
    var x2 = x.length > 1 ? ',' + x[1] : '';
    var rgx = /(\d+)(\d{3})/;
    while (rgx.test(x1)) {
        x1 = x1.replace(rgx, '$1 $2');
    }
    return x1 + x2;
}

for (i = 0; i < tests.length; i++)
{
  alert(addCommas(tests[i]) + '<br>');
}