# Calculate the mod of a collection index

Wrap an index into 0…length−1, also for negative numbers.

Canonical: https://1loc.dev/number/calculate-the-mod-of-collection-index/
Version: 1.0.0
Runtime: JavaScript · Node.js 22 / modern browsers
Verification: Local fixtures; no external services
Updated: 2026-10-28

## Implementation

```javascript
function mod({ index, length }) { return ((index % length) + length) % length; }
```

## Input

```json
{
  "index": -1,
  "length": 5
}
```

## Expected output

```json
4
```

## Edge case

Input:
```json
{
  "index": 12,
  "length": 5
}
```
Output:
```json
2
```

## Limits

- JavaScript’s % keeps the sign of the dividend (-1 % 5 is -1); adding length first fixes it.

The lab validates types and size before calling this function. Add equivalent validation at your own trust boundary.

## Source and license

Original: https://github.com/phuocng/1loc/blob/14b481cf60dff1620ab1bb99a43b1f68068cdd9b/contents/calculate-the-mod-of-collection-index.mdx
Author: Nguyen Huu Phuoc and original contributors
Contributors: marcobiedermann
Source commit: 14b481cf60dff1620ab1bb99a43b1f68068cdd9b
Copyright (c) 2020 phuoc-ng
License: MIT — https://1loc.dev/licenses/
Maintainer: Oveyon
Modifications: Takes index and length as one object. Original MDX retained as inert source evidence.