JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="inputText" /><br>
<input type="button" id="button" value="Format Currency" />
<hr />
<div id="output"></div>

JavaScript

function addCommas(inputText) {
    // pattern works from right to left
	var commaPattern = /(\d+)(\d{3})(\.\d*)*$/;
	var callback = function (match, p1, p2, p3) {
		return p1.replace(commaPattern, callback) + ',' + p2 + (p3 || '');
	};
	return inputText.replace(commaPattern, callback);
}

// events for demo
$(function(){var $inputText = $('#inputText');
    var $button = $('#button');
    var $output = $('#output');
    
    $button.click(function(){
        $output.html(addCommas($inputText.val()));
    });
});