Check if a URL is absolute

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

JavaScript Version 1.0.0 · MIT · .md

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

Input & expected output

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

true

The edge case

"//cdn.example.com/app.js"

false

Know the 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 function above is the implementation used by the lab. The lab validates JSON types and caps size before execution; include equivalent validation when accepting untrusted input in your application.

Keep exploring

• Capitalize a string — Uppercase the first Unicode code point and preserve the remaining text.
• Reverse a string — Reverse Unicode code points without splitting surrogate pairs.
• Check a palindrome — Compare a string with its code-point reversal.