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

```js
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;
}
```

Basically, we calculate how many commas we will need to add (`Math.floor((String(amount).length-1)/3)`). If the `amount` is 3 digits, we need 0 commas, since `floor((3-1)/3) = floor(2/3) = 0`. If the `amount` is 7 digits, we need 2 commas, since `floor((7-1)/3) = floor(6/3) = 2`. And so on.

Then, we do a for loop with that number, and insert our commas, *starting from the back*. We find the position where we need to split the string in half, and then insert a comma in between the two halves of the string.

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. Remember, we are inserting commas starting from the back of the string, so we are subtracting to offset, not adding.

![Demo](/images/commas.gif)

Here is a version that can handle decimals and invalid inputs:

```js
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;
}
```