format.js

function formatNumber(number) {
  let format = [];
  let num = parseFloat(number);
  let decimal = (Math.abs(num) % 1).toString().slice(0,4);
  let isNegative = parseInt(num) < 0;
  let hasDecimal = number.indexOf('.') !== -1;
  
  if (isNegative) {
    format.push('-');
  }
  
  number = Math.floor(Math.abs(num))
    .toString()
    .split('')
    .reverse()
    .join('')
    .match(/.{1,3}/g)
    .reverse()
    .join(',');
  format.push(number.toString());

  if (hasDecimal) {
     format.push('.', decimal.toString().slice(2,4));
  } else {
    format.push('.00');
  }
  
  return format.join('');
}


console.log(formatNumber("123456789.59"))
console.log(formatNumber("456789.59"))
console.log(formatNumber("-89.59"))
console.log(formatNumber("9.59"))