# Check if a URL is absolute

Detect a scheme such as https: at the start of a URL string.

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

## Implementation

```javascript
function isAbsoluteUrl(url) { return /^[a-z][a-z0-9+.-]*:/i.test(url); }
```

## Input

```json
"https://api.oveyon.com/v1/send"
```

## Expected output

```json
true
```

## Edge case

Input:
```json
"//cdn.example.com/app.js"
```
Output:
```json
false
```

## Limits

- A scheme is not a safety check: javascript: and data: URLs are absolute too. Allow-list https: before storing or following a URL, such as a webhook endpoint.
- Schemes are case-insensitive (RFC 3986), so this version adds the i flag the original regex lacked.

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-url-is-absolute.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 adds the case-insensitive flag (RFC 3986 schemes); explicit input bounds, edge case and scheme-safety note. Original MDX retained as inert source evidence.