JSFiddle - React, Tailwind, and code Playground

HTML

<p>
  Formatting the number <span id="input">-1654349.7</span> as money (USD).
</p>
<p>
  Result: <span id="result"></span>
</p>
<p>
  Result with defaults: <span id="defaults"></span>
</p>

CSS

span {
  font-weight: bold;
}

JavaScript

/* 
decimals: number of decimal places to output, defaults to 2 when omitted
decimal_sep: character used as decimal separtor, it defaults to '.' when omitted
thousands_sep: char used as thousands separator, it defaults to ',' when omitted
*/
Number.prototype.toMoney = function(decimals, decimal_sep, thousands_sep)
{ 
   var n = this,
   c = isNaN(decimals) ? 2 : Math.abs(decimals), //if decimal is zero we must take it, it means user does not want to show any decimal
   d = decimal_sep || '.', //if no decimal separator is passed we use the dot as default decimal separator (we MUST use a decimal separator)

   /*
   according to [https://stackoverflow.com/questions/411352/how-best-to-determine-if-an-argument-is-not-sent-to-the-javascript-function]
   the fastest way to check for not defined parameter is to use typeof value === 'undefined' 
   rather than doing value === undefined.
   */   
   t = (typeof thousands_sep === 'undefined') ? ',' : thousands_sep, //if you don't want to use a thousands separator you can pass empty string as thousands_sep value

   sign = (n < 0) ? '-' : '',

   //extracting the absolute value of the integer part of the number and converting to string
   i = parseInt(n = Math.abs(n).toFixed(c)) + '', 

   j = ((j = i.length) > 3) ? j % 3 : 0; 
   return sign + (j ? i.substr(0, j) + t : '') + i.substr(j).replace(/(\d{3})(?=\d)/g, "$1" + t) + (c ? d + Math.abs(n - i).toFixed(c).slice(2) : ''); 
}

// Use it.
var amount = document.getElementById('input').innerHTML;
document.getElementById('result').innerHTML = Number(amount).toMoney(2, ".", "");
document.getElementById('defaults').innerHTML = Number(amount).toMoney();