# Check a leap year

Apply the Gregorian divisible-by-4, century and 400-year rules.

Canonical: https://1loc.dev/validator/check-if-a-year-is-leap-year/
Version: 1.0.0
Runtime: JavaScript · Node.js 22 / modern browsers
Verification: Local fixtures; no external services
Updated: 2026-10-28

## Implementation

```javascript
function isLeapYear(year) { return year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0); }
```

## Input

```json
2024
```

## Expected output

```json
true
```

## Edge case

Input:
```json
1900
```
Output:
```json
false
```

## Limits

- Input is an integer year from 1 to 9,999 in the proleptic Gregorian calendar.

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/check-if-a-year-is-leap-year.mdx
Author: Nguyen Huu Phuoc and original contributors
Contributors: See original repository history
Source commit: 14b481cf60dff1620ab1bb99a43b1f68068cdd9b
Copyright (c) 2020 phuoc-ng
License: MIT — https://1loc.dev/licenses/
Maintainer: Oveyon
Modifications: Curated implementation; explicit input bounds, edge cases, local fixtures and rebuilt presentation. Original MDX retained as inert source evidence.