# Check if a given integer is a prime number

Trial division up to the square root, without allocating an array.

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

## Implementation

```javascript
function isPrime(n) { if (n < 2) return false; for (let i = 2; i * i <= n; i++) if (n % i === 0) return false; return true; }
```

## Input

```json
97
```

## Expected output

```json
true
```

## Edge case

Input:
```json
1
```
Output:
```json
false
```

## Limits

- The original built an array of candidates; a loop stops at the first divisor and allocates nothing.
- Fine for numbers up to the lab’s one billion; use a sieve for many queries.

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-given-integer-is-a-prime-number.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: Replaced the candidate array with an allocation-free loop. Original MDX retained as inert source evidence.