# Adding Commas to Numbers

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:

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.

Here is a version that can handle decimals (keep in mind that Javascript does cut off decimals after a certain point), negative numbers and invalid inputs:

if (isNaN(Number(amount))) {

return amount;

}

let negative = amount < 0;

amount = Math.abs(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];

}

if (negative) {

amount_mod = `-${amount_mod}`;

}

return amount_mod;

}

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 (keep in mind that Javascript does cut off decimals after a certain point), negative numbers and invalid inputs:

```js

function format_commas(amount) {

if (isNaN(Number(amount))) {

return amount;

}

let negative = amount < 0;

amount = Math.abs(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];

}

if (negative) {

amount_mod = `-${amount_mod}`;

}

return amount_mod;

}

```