Code Bug Fix: How to specify a custom language parser alias for rouge in Jekyll 3?

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How does one configure Jekyll & Rouge to specify that one language
should be highlighted using the parser of another language.

For example, I want to be able to do this in my markdown source files:

> foo();
Uncaught ReferenceError: foo is not defined

… but have that code block syntax highlighted using the same
syntax highlighter Javascript language parser.

I want to do this for a couple of reasons:

  1. Semantic correctness in the source files
  2. Be able to run tools such as prettier and have them bypass these code blocks

Rouge already does have the concept of language aliases (see example below),
however is it possible to specify a custom language alias via Jekyll,
and if so how can this be done?



I am using Jekyll 3.8.5 with Rouge 3.11.0.


The following is the relevant part of my Jekyll config file:

highlighter: rouge


So as to be really clear about what “language aliases” refers to,
I’ll provide an example:

For Javascript, you can use both js and javascript after the code fences,
as they are language aliases by default in Rouge.
Thus, the following two code blocks are identical:

Using language alias js:


Using language alias javascript:


I was able to “extend” the Rogue highlighter by utilizing Jekyll’s _plugins directory.

Much of my research came from rogue-ruby‘s GitHub Issue #1392 and the source code for LinkedIn’s rest-li jekyll-based website.


  1. Create a _plugins directory in your Jekyll root directory
  2. Create a ruby file in the _plugins directory
  3. Use the following code to inherit the existing Rogue Javascript code and “extend” (aka overwrite) the list of aliases.
# This "hook" is executed right before the site's pages are rendered
Jekyll::Hooks.register :site, :pre_render do |site|
  puts "Adding more JavaScript Markdown aliases..."
  require "rouge"

  # This class defines the PDL lexer which is used to highlight "pdl" code snippets during render-time
  class MoreJSLexer < Rouge::Lexers::Javascript
    title 'MoreJS'
    aliases 'js', 'nodejs-repl'
  1. Run jekyll serve and note the custom log line "Adding more JavaScript Markdown aliases..."
 Incremental build: disabled. Enable with --incremental
       Jekyll Feed: Generating feed for posts
Adding more JavaScript Markdown aliases...
                    done in 0.637 seconds.
 Auto-regeneration: enabled for '/Users/kueng/work/sandboxes/minima'
    Server address:
  Server running... press ctrl-c to stop.


I used the minima GitHub repository as my sandbox and jekyll 3.8.7. I created a _plugins/more_javascript.rb Ruby file using the code in step 3. I edited one of the markdown files with the markdown snippet below and noticed only js and nodejs-repl had syntax highlighting.

var a, b, c;
a = 5; b = 6; c = a + b;
document.getElementById("demo1").innerHTML = c;
var a, b, c;
a = 5; b = 6; c = a + b;
document.getElementById("demo1").innerHTML = c;
var a, b, c;
a = 5; b = 6; c = a + b;
document.getElementById("demo1").innerHTML = c;

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