JSFiddle - React, Tailwind, and code Playground

HTML

<div id="in">1000000000.9</div>
<div id="out"></div>

JavaScript

String.prototype.reverse = function() {
    var s = "";
    var i = this.length;
    while (i>0) {
        s += this.substring(i-1,i);
        i--;
    }
    return s;
};

var n = $("#in").text().reverse().replace(/(\d{3})/g, "$1 ").reverse();
$("#out").text(n);