How to add Commas in Numbers

I had to add commas to a number in Javascript today. Thought it was kinda interesting, so here is what I came up with:

function format_commas(amount) {
  let amount_mod = String(amount);
  //iterate the amount of commas there are
  for (let i=0; i < Math.floor((String(amount).length-1)/3); i++) {
    let position = amount_mod.length-3*(i+1)-i;
    amount_mod = amount_mod.substring(0, position)+","+amount_mod.substring(position, amount_mod.length);
  }
  return amount_mod;
}

I think the most interesting part of this code was the 5th line (let position = amount_mod.length-3*(i+1)-i;). You might be wondering with the -i at the end is necessary. That's there because we are increasing the string's length by adding a comma, so we need to offset it.

Here is a version that can handle decimals, undefined and NaN:

function format_commas(amount) {
  if (isNaN(Number(amount))) {
    return amount;
  }
  let before_dec = String(amount).split('.')[0];
  let amount_mod = before_dec;
  //iterate the amount of commas there are
  for (let i=0; i < Math.floor((before_dec.length-1)/3); i++) {
    let position = amount_mod.length-3*(i+1)-i;
    amount_mod = amount_mod.substring(0, position)+","+amount_mod.substring(position, amount_mod.length);
  }
  if (String(amount).split('.')[1]) {
    amount_mod = amount_mod+"."+String(amount).split('.')[1];
  }
  return amount_mod;
}