@fnix/lexxy-mathjax

LaTeX equations for Lexxy (the rich text editor for Rails), rendered with MathJax.

• Toolbar button (and Cmd/Ctrl+Shift+E) opens a dialog with a LaTeX input and live preview
• Click any equation to edit it
• Inline (e^{i\pi} + 1 = 0 in a sentence) and display/block modes
• Saved HTML stores the raw LaTeX in a data-latex attribute, so content round-trips cleanly and is typeset again on display pages

How it works

Equations are stored in the document (and in what ActionText persists) as:

<span class="lexxy-math" data-latex="\frac{a}{b}">\frac{a}{b}</span>   <!-- inline -->
<div  class="lexxy-math" data-latex="\int_0^1 x\,dx">...</div>         <!-- display/block -->

Inside the editor the extension typesets these with the page's MathJax. MathJax is host-provided, not bundled: load MathJax v3 (tex-chtml) on any page that uses the editor or displays saved content.

Installation

With importmap-rails (no build)

Vendor this package (or pin it) and pin lexxy's bare specifier so the extension resolves it:

# config/importmap.rb
pin "lexxy"                                        # from the lexxy gem, per lexxy's install docs
pin "@37signals/lexxy", to: "lexxy.js"             # alias used by @fnix/lexxy-mathjax
pin "@fnix/lexxy-mathjax", to: "lexxy-mathjax/index.js"  # vendored src/ of this package
# ...pin the files under src/ as well if vendoring, e.g. via pin_all_from

With a JS bundler (esbuild / vite / webpack)

npm install @fnix/lexxy-mathjax @37signals/lexxy

Register the extension

import { configure } from "@37signals/lexxy"
import MathjaxExtension from "@fnix/lexxy-mathjax"

configure({
  global: { extensions: [ MathjaxExtension ] }
})

Include the stylesheet (@fnix/lexxy-mathjax/styles, or copy styles/lexxy-mathjax.css into your assets).

Load MathJax

<script>
  window.MathJax = { chtml: { displayAlign: "center" } }
</script>
<script async src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-chtml.js"></script>

If MathJax is missing, the editor still works and equations show their raw LaTeX.

Rails / ActionText integration

1. Allow data-latex through the server-side sanitizer (ActionText strips unknown attributes when rendering):

# config/initializers/lexxy_mathjax.rb
ActiveSupport.on_load(:action_text_content) do
  ActionText::ContentHelper.allowed_attributes += [ "data-latex" ]
end

2. Typeset saved content on display pages. Load MathJax (as above) plus a small script that renders [data-latex] elements, Turbo-compatible:

function typesetEquations() {
  const elements = document.querySelectorAll(".trix-content [data-latex], .lexxy-content [data-latex]")
  if (elements.length === 0 || !window.MathJax?.typesetPromise) return

  elements.forEach((el) => { el.textContent = el.getAttribute("data-latex") })
  window.MathJax.typesetPromise([ ...elements ])
}

document.addEventListener("turbo:load", typesetEquations)
document.addEventListener("DOMContentLoaded", typesetEquations)

Note: typesetPromise scans for \(...\) delimiters by default; the snippet above instead sets each element's content from data-latex and typesets just those elements. If you prefer delimiter scanning, configure tex.inlineMath accordingly.

Demo

Try it online: https://fnix.github.io/lexxy-mathjax/

Or serve the repo statically and open the demo locally:

npx serve .
# then visit http://localhost:3000/demo/

Tests

npm install
npm test

Development notes

The code follows lexxy's own conventions: plain ES modules, vanilla JS, no build step. Structure:

• src/nodes/math_node.js — DecoratorNode storing { latex, display }; createDOM typesets with MathJax, exportDOM/importDOM handle the data-latex HTML format
• src/extensions/mathjax_extension.js — the Lexxy.Extension: registers the node, the insertMath command, the toolbar button, and click-to-edit
• src/elements/math_editor_dialog.js — <lexxy-math-editor> dialog with live preview
• src/helpers/mathjax_helper.js — serialized MathJax typesetting with graceful degradation