JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" onkeyup="addCommas(this.value);" /> <br /><br />
<label id="result">Result: </label>

JavaScript

function addCommas(nStr)
{
	nStr += '';
	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');
	}
	var result = x1 + x2;
    document.getElementById('result').innerHTML = result;
}