From 3618aa63806c470eee0be780f6afee243fcc0588 Mon Sep 17 00:00:00 2001
From: Abhishek Chauhan <abhishek.chauhan@sonatype.com>
Date: Sat, 6 Jun 2026 22:54:33 -0500
Subject: [PATCH] fix: treat null mappings from extend() as character removal

extend({ '%': null }) worked in 1.6.5 to drop a character (e.g. "100%"
-> "100"), but since 1.6.6 it throws "TypeError: Cannot read properties
of null (reading 'replace')": the charMap lookup yields null, the two
`=== undefined` fallbacks don't catch it, and null.replace() throws.

Coerce a null mapping to an empty string so the character is removed,
restoring the earlier behavior and preventing the crash.

Fixes #177
---
 slugify.js      |  2 ++
 test/slugify.js | 12 ++++++++++++
 2 files changed, 14 insertions(+)

diff --git a/slugify.js b/slugify.js
index f5a6a1e..0befa5e 100644
--- a/slugify.js
+++ b/slugify.js
@@ -36,6 +36,8 @@
         var appendChar = locale[ch];
         if (appendChar === undefined) appendChar = charMap[ch];
         if (appendChar === undefined) appendChar = ch;
+        // A null mapping (e.g. extend({ '%': null })) removes the character.
+        if (appendChar === null) appendChar = '';
         if (appendChar === replacement) appendChar = ' ';
         return result + appendChar
           // remove not allowed characters
diff --git a/test/slugify.js b/test/slugify.js
index e2581d2..0635c26 100644
--- a/test/slugify.js
+++ b/test/slugify.js
@@ -285,4 +285,16 @@ describe('slugify', () => {
     delete require.cache[require.resolve('../')]
     slugify = require('../')
   })
+
+  it('removes characters mapped to null via extend()', () => {
+    // https://github.com/simov/slugify/issues/177
+    slugify.extend({ '|': null, '%': null, $: null })
+    t.equal(slugify('100%'), '100')
+    t.equal(slugify('a|b$c'), 'abc')
+
+    delete require.cache[require.resolve('../')]
+    slugify = require('../')
+
+    t.equal(slugify('100%'), '100percent')
+  })
 })