MathJax v4/v3 support: plugin emits v2 API config (MathJax.Hub.Config) regardless of loaded version

[cptkoolbeenz]

Hi — thanks for maintaining this plugin. I'm reporting a latent bug that surfaces when the kblog_mathjax_custom_location override is used to load MathJax v3 or v4 from a CDN. I hit this while auditing a production WordPress site that's been on v4 for a while.

Summary

When kblog_mathjax_custom_location is set to a v3/v4 MathJax URL (e.g. https://cdnjs.cloudflare.com/ajax/libs/mathjax/4.0.0/tex-chtml.min.js), the plugin loads the v4 runtime successfully, but:

1. class-mathjax-latex.php:223 unconditionally emits wp_add_inline_script('mathjax', 'MathJax.Hub.Config(' . wp_json_encode($mathjax_config) . ');') whenever the mathjax_config filter returns a non-empty array. MathJax.Hub does not exist in v3 or v4 — it was removed. The inline script throws TypeError: MathJax.Hub is undefined / ReferenceError: MathJax is not defined on every page load where the shortcode is present and a mathjax_config filter is registered.

2. class-mathjax-latex.php::script_loader_tag() rewrites inline scripts on the mathjax handle from <script type='text/javascript'> to <script type='text/x-mathjax-config'>. That type is v2-specific — MathJax v3/v4 ignores it — so even the replacement wouldn't run.

3. add_query_arg('config', $config, $mathjax_location) appends ?config=default to the script URL. v4 ignores this (its configuration model is window.MathJax = {...} set before the script loads), and for sites pointing at tex-chtml.min.js the query string is noise.

Visual rendering still works on v4-configured sites because v4's auto-typesetter runs independently of any Hub.Config() call — so the bug is often invisible to users but pollutes DevTools on every page load, and any customization registered via the mathjax_config filter is silently dropped on v4.

Reproduction

1. Install this plugin on a site with MathJax enabled.
2. In Settings → MathJax, set the custom location to https://cdnjs.cloudflare.com/ajax/libs/mathjax/4.0.0/tex-chtml.min.js (or any v3/v4 URL).
3. Register a mathjax_config filter that returns any non-empty array, e.g.:

   add_filter('mathjax_config', fn() => ['TeX' => ['equationNumbers' => ['autoNumber' => 'AMS']]]);

4. Visit any page containing the [mathjax] shortcode and open DevTools → Console.
5. Observe: ReferenceError: MathJax is not defined at the emitted MathJax.Hub.Config({...}) inline script.

Suggested fix

Detect the loaded version from the script URL and branch config emission:

• If URL matches a v3/v4 pattern (e.g. /mathjax/[34]\., or ends with tex-chtml, tex-mml-chtml, tex-svg, etc.):
  • Emit window.MathJax = {...} as an inline script with position 'before' (so it runs before the library loads and is picked up by v4's config reader).
  • Translate v2 config keys to v4 equivalents where possible (TeX.equationNumbers.autoNumber: 'AMS' → tex.tags: 'ams', tex2jax.inlineMath → tex.inlineMath, etc.).
  • Skip the script_loader_tag type rewrite for v3/v4.
  • Don't append ?config=default to the URL.
• Otherwise, keep the existing v2 behavior (fully backward compatible).

A helper is_mathjax_v4($url) of ~5 lines handles the detection. The branching in add_script() and script_loader_tag() is maybe 20 lines total. The v2→v4 config key translation can start minimal (just tex.tags and tex.inlineMath / tex.displayMath) and expand over time — or just emit the raw dict under a tex key and let the filter author write v4-shape config directly.

Workaround for site owners

Until upstream has a fix, site owners on v4 can avoid the error by either:

• Not registering any mathjax_config filter (the plugin emits nothing if the filter returns empty)
• Emitting their own window.MathJax = {...} via wp_head instead of the filter

Offer

Happy to submit a PR if that would be useful. Let me know if there's a preferred approach for the v2→v4 config translation (full mapping vs pass-through vs something in between) and I'll match that.

Cheers!

[paulschreiber]

Is it worth dropping v2 support and defaulting to v4?