Bug Report: Emoji Variation Selectors Not Removed from Slugs

[comanche]

Bug Report: Emoji Variation Selectors Not Removed from Slugs

Description

The slug() function does not fully remove emojis that contain Unicode variation selectors (U+FE00 to U+FE0F), leaving invisible characters in the generated slug. This creates broken anchor links.

Expected Behavior

Based on the test fixtures, emojis should be completely removed from slugs:

• "😄 unicode emoji" → "-unicode-emoji" ✓

Actual Behavior

Emojis composed with variation selectors leave the invisible variation selector character in the slug:

• "🗃️ File Cabinet" → "️-file-cabinet" ✗ (contains invisible U+FE0F)

Reproduction

const { slug } = require('github-slugger');

// Simple emoji - works correctly
console.log(slug('✅ Checkmark'));
// Output: "-checkmark" ✓

// Emoji with variation selector - leaves invisible character
console.log(slug('🗃️ File Cabinet'));
// Output: "️-file-cabinet" ✗ (starts with invisible U+FE0F)

// Inspect the bytes to see the problem
const result = slug('🗃️ File Cabinet');
console.log('Bytes:', Buffer.from(result).toString('hex'));
// Output: "efb88f2d66696c652d636162696e6574"
//          ^^^^^^ = U+FE0F (variation selector - should not be here!)

Root Cause

Some emojis are composed of multiple Unicode characters:

1. Base emoji character (e.g., U+1F5C3 = 🗃)
2. Variation Selector-16 (U+FE0F) - makes the emoji display in color

Examples:

• 🗃️ = U+1F5C3 + U+FE0F (2 characters)
• 1️⃣ = U+0031 + U+FE0F + U+20E3 (3 characters)
• ✅ = U+2705 (1 character) ✓

The current implementation removes the visible emoji character but leaves the invisible variation selector behind.

Impact

This breaks markdown anchor links in real-world usage:

## 🗃️ File Management

Link to section: [See File Management](#-file-management)

The link won't work because the slug contains an invisible character that doesn't match.

Affected Emojis

Common emojis with variation selectors include:

• 🗃️ (File Cabinet)
• ☑️ (Check Box)
• ✔️ (Check Mark)
• ⭐️ (Star)
• ❤️ (Red Heart)
• 1️⃣ 2️⃣ 3️⃣ (Keycap Numbers)
• Many others with U+FE0F modifier

Proposed Fix

Strip variation selectors (U+FE00 through U+FE0F) after processing emojis:

function slug(value) {
  // ... existing slug generation code ...

  // Remove variation selectors that may remain after emoji removal
  result = result.replace(/[\uFE00-\uFE0F]/g, '');

  return result;
}

Workaround

Users can post-process the slug output:

const { slug } = require('github-slugger');
const result = slug('🗃️ File Cabinet').replace(/[\uFE00-\uFE0F]/g, '');
// Output: "-file-cabinet" ✓

Environment

• github-slugger version: 2.0.0 (latest)
• Node.js version: v20+
• Platform: All platforms

Additional Context

Similar issues have been addressed in related libraries:

• emoji-regex #45 - variation selector handling
• emojibase #1 - variation selector capture

This is becoming more common as modern emoji keyboards automatically add variation selectors for better rendering.

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Would you like me to submit a pull request with the fix?

[wooorm]

this project follows how github works, so whether this is a bug depends on what github does; see the build scripts for more info; it can also be that GH now uses modern unicode, see references to unicode-13.0.0, in which case those references can be updated