Compile Ruby Extensions to JavaScript

On this page, you’ll learn how to compile an Asciidoctor extension written in Ruby to JavaScript using Opal.

This is an advanced technique that has some limitations. The recommended way to use extensions in Asciidoctor.js is to write them directly in JavaScript.

Now that you’ve been warned, let’s say you want to use the following Asciidoctor extension (written in Ruby) in your JavaScript based application.

# An inline macro that turns URIs with the tel: URI scheme into links.
# Usage
#   tel:1-800-555-1212[]
Asciidoctor::Extensions.register do
  inline_macro do
    named :tel
    parse_content_as :text
    process do |parent, target, attrs|
      if (text = attrs['text']).empty?
        text = target
      target = %(tel:#{target})
      (create_anchor parent, text, type: :link, target: target)


To compile this extension, you will need to install the opal-compiler package:

$ npm install opal-compiler

Once this package is installed, you can write the following Node script to compile the extension:

const fs = require('fs')
const Builder = require('opal-compiler').Builder
// Opal object will be available on the global scope

const builder = Builder.create()
builder.appendPaths('lib') (1)
const result ='tel-inline-macro.rb') (2)
fs.writeFileSync('tel-inline-macro.js', result.toString(), 'utf8') (3)
1 The extension file is in the lib directory
2 The extension file is named tel-inline-macro.rb
3 A file named tel-inline-macro.js will be written

Then run it:

$ node compile.js

The above command will create a file named tel-inline-macro.js containing the extension code in JavaScript.


In the previous section, we’ve showed how to compile a Ruby extension to JavaScript. In this section we will use this extension.

Let’s create a simple document:

= Contacts

== Professional

Guillaume Grossetie tel:1-800-555-1212[]

Next we create a Node script to convert this document:

const asciidoctor = require('asciidoctor')()
require('./tel-inline-macro.js') (1)

const html = asciidoctor.convertFile('document.adoc', { 'to_file': false }) (2)
1 Load the local extension named tel-inline-macro.js
2 Convert the document with the tel inline macro extension enabled
Asciidoctor.js must be installed: npm install asciidoctor.js

Then we run it:

$ node convert.js
<div class="sect1">
<h2 id="_professional">Professional</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Guillaume Grossetie <a href="tel:1-800-555-1212">1-800-555-1212</a></p> (1)
1 The tel macro was converted to a link