fix: fall back to default checkbox renderer when extension returns false

[sarathfrancis90]

Markdown flavor: GitHub Flavored Markdown

A renderer extension named after a built-in token can return false to fall back to the default renderer — that's the documented contract in USING_PRO, and it works for code, heading, text and the rest. It doesn't work for checkbox: the checkbox silently disappears instead.

const marked = new Marked({
  extensions: [{
    name: 'checkbox',
    renderer(token) {
      return token.checked ? false : '<span class="todo"></span> ';
    },
  }],
});

marked.parse('- [x] one\n- [ ] two\n');

I expected the checked item to fall back to the default <input checked="" disabled="" type="checkbox">, but it renders as just <li>one</li>. The unchecked item never returns false, and it renders fine, so only the fallback path is broken.

Parser decides whether a false return should fall through to the built-in renderer by testing the token type against a hardcoded list of built-in type names. checkbox was added to both parse switches in #3755 but never to either list, so the fallback branch is unreachable for it and the ret || '' path wins.

Added checkbox to the block and inline lists. Both matter in practice: a tight list puts the checkbox token at block level while a loose list puts it inline, so there's a test for each.

Contributor

• Tests added (checkbox extension fallback, tight and loose list)
• No new feature to document

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Project	Deployment	Actions	Updated (UTC)
marked-website	Ready	Preview, Comment	Jul 18, 2026 2:31pm

[UziTech]

Nice catch! 💯