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; }