JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" onblur="javascript:formatta(this)" />

JavaScript

window.formatta = function(casella){
    
    if (casella.value == "") return;
    casella.value = formattaValuta(casella.value);    
}

//script trovato qui: http://www.naprej.com/format_currency_euro_in_javascript_
window.formattaValuta = function (number)
{
	var numberStr = parseFloat(number).toFixed(2).toString();
	var numFormatDec = numberStr.slice(-2); /*decimal 00*/
	numberStr = numberStr.substring(0, numberStr.length-3); /*cut last 3 strings*/
	var numFormat = new Array;
	while (numberStr.length > 3) {
		numFormat.unshift(numberStr.slice(-3));
		numberStr = numberStr.substring(0, numberStr.length-3);
	}
	numFormat.unshift(numberStr);
	return numFormat.join('.')+','+numFormatDec; /*format 000.000.000,00 */
}