# Check if a character is a digit

Test that a string is exactly one ASCII digit from 0 to 9.

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

## Implementation

```javascript
function isDigit(char) { return /^[0-9]$/.test(char); }
```

## Input

```json
"7"
```

## Expected output

```json
true
```

## Edge case

Input:
```json
" "
```
Output:
```json
false
```

## Limits

- The original `char < 10` accepted "", " " and "-1" because of numeric coercion, and one variant referenced an undefined variable. This version tests exactly one ASCII digit.
- Other Unicode digits, such as Arabic-Indic numerals, return false.

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-character-is-a-digit.mdx
Source file (this edition): https://github.com/akamind/1loc/blob/0a77f131b22d2ba3b7ec1728846f900c22821678/contents/check-if-a-character-is-a-digit.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 coercion-based variants (which accepted "", " " and "-1", and one referenced an undefined variable) with an exact ASCII digit test. Original MDX retained as inert source evidence.