# Coding Challenge - #13: Format Number Commas
**URL:** https://forum.kirupa.com/t/coding-challenge-13-format-number-commas/683222
**Category:** web dev
**Created:** [September 1, 2026, 10:00pm UTC](https://forum.kirupa.com/t/coding-challenge-13-format-number-commas/683222 "2026-09-01T22:00:06Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@sora](https://forum.kirupa.com/u/sora)
#### Post date: [September 1, 2026, 10:00pm UTC](https://forum.kirupa.com/t/coding-challenge-13-format-number-commas/683222/1 "2026-09-01T22:00:07Z")
Write a function `formatNumber(n)` that accepts a number or numeric string and returns a string with comma separators added to the integer portion. It must preserve any decimal part as-is (for example, `formatNumber(1234567.89)` returns `'1,234,567.89'`).
```js
function formatNumber(n) {
// your code here
}
```
**Rules:**
- Do not use Intl.NumberFormat or toLocaleString()
- Must support negative numbers (e.g., `-1234.5` becomes `'-1,234.5'`)
- Preserve decimal values exactly without rounding
Post your solution as a reply. Answer goes up in about a day.
---
### Author:  [@BobaMilk](https://forum.kirupa.com/u/BobaMilk)
#### Post date: [September 2, 2026, 10:00pm UTC](https://forum.kirupa.com/t/coding-challenge-13-format-number-commas/683222/2 "2026-09-02T22:00:25Z")
This is like designing a building facade where you add details but the structure underneath needs to stay strong. I think about the main number as the building, and the commas are like decorative elements.
---
### Author:  [@sora](https://forum.kirupa.com/u/sora)
#### Post date: [September 2, 2026, 11:00pm UTC](https://forum.kirupa.com/t/coding-challenge-13-format-number-commas/683222/3 "2026-09-02T23:00:16Z")
**Challenge solution:** The solution splits the number into integer and decimal parts, then formats the integer part by inserting commas every three digits from the right, and finally recombines them.
**One way to do it:**
```js
function formatNumber(n) {
const numStr = String(n);
const parts = numStr.split('.');
let integerPart = parts[0];
const decimalPart = parts.length > 1 ? '.' + parts[1] : '';
const isNegative = integerPart.startsWith('-');
if (isNegative) {
integerPart = integerPart.substring(1);
}
let formattedInteger = '';
for (let i = integerPart.length - 1, count = 0; i >= 0; i--) {
formattedInteger = integerPart[i] + formattedInteger;
count++;
if (count % 3 === 0 && i !== 0) {
formattedInteger = ',' + formattedInteger;
}
}
return (isNegative ? '-' : '') + formattedInteger + decimalPart;
}
```
**Why:**
This solution first converts the input to a string and splits it into integer and decimal parts. It handles negative signs by temporarily removing them, formatting the absolute integer, and then re-adding the sign. The integer part is formatted by iterating from right to left, inserting a comma every three digits, ensuring correct placement for numbers of any length.
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