Get the file extension from a file name

Return the text after the last dot, or an empty string.

JavaScript Version 1.0.0 · MIT · .md

function fileExtension(name) { const dot = name.lastIndexOf('.'); return dot > 0 ? name.slice(dot + 1) : ''; }

Input & expected output

"invoice-8812.pdf"

"pdf"

The edge case

".env"

""

Know the limits

• The original returned the whole name when there was no dot. A leading dot marks a hidden file, not an extension.
• Only the last part is returned: archive.tar.gz gives gz.

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.