fix: treat null mappings from extend() as character removal

[abhu85]

Fixes #177

Problem

extend({ '%': null }) was used in 1.6.5 to make a character drop out of the slug (e.g. "100%" → "100"). Since 1.6.6 the same call throws:

TypeError: Cannot read properties of null (reading 'replace')

In the reduce that builds the slug, the lookup resolves to null:

var appendChar = locale[ch];
if (appendChar === undefined) appendChar = charMap[ch]; // -> null (from extend)
if (appendChar === undefined) appendChar = ch;          // null !== undefined, so stays null
...
return result + appendChar.replace(/* ... */)           // null.replace(...) throws

Fix

Coerce a null mapping to an empty string, so the character is removed — restoring the pre-1.6.6 behavior and preventing the crash:

if (appendChar === null) appendChar = '';

undefined mappings are unchanged (they still fall back to the original character), and string/empty-string mappings are unaffected.

Tests

Added a regression test (mirrors the existing extend() tests, with the usual require.cache reset):

slugify.extend({ '|': null, '%': null, $: null })
t.equal(slugify('100%'), '100')
t.equal(slugify('a|b$c'), 'abc')

Verified it fails without the fix (the reported TypeError) and passes with it. Full suite: 46 passing.

[Trott]

This change seems OK to me, but also as an FYI: If you want to switch to the slug module, which would likely be a drop-in replacement (if you're using ESM), it (1) doesn't do magic with special characters like % so it will do what you are trying to achieve with extend({'%': null}) by default, and (2) handles extend({'%': null}) in the way you expect.

import slug from 'slug';
import slugify from 'slugify';

// Default behavior of slug() is already what you want.
console.log(slug('100%')); // 100
console.log(slugify('100%')); // 100percent

// Extend works the same way for both libraries.
slug.extend({ '%': ' percent' })
slugify.extend({ '%': ' percent' })
console.log(slug('100%')); // 100-percent
console.log(slugify('100%')); // 100-percent

// Null currently throws in slugify, but works as expected in slug.
slug.extend({ '%': null })
slugify.extend({ '%': null })

console.log(slug('100%')); // 100
console.log(slugify('100%')); // Throws TypeError

All that said, I like this code change. For selfish reasons, I'm in favor of anything that brings slugify() more in alignment with slug().