Sprockets 3.6 CoffeeScript processor with sourcemaps.

.blade.yml

load_paths:
  - src

logical_paths:
  - simple.js

build:
  logical_paths:
    - simple.js
  path: dist

coffee_script_processor.rb

# For sprockets 3.6.0, as required by blade 0.7.0, as required by trix 1.3.1

# Inspired by UglifyWithSourceMaps
# https://blog.experteer.engineering/generating-sourcemaps-with-sprockets-3-and-uglify.html

# Google Doc SourceMaps Specification
# https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit#

# FIXME
#   - How to register a different CoffeeScript processor with Sprockets?  Does this exist:
#     `Sprockets.register_processor(...)`

require 'sprockets/digest_utils'
require 'sprockets/autoload'

module Sprockets
  # Processor engine class for the CoffeeScript compiler.
  # Depends on the `coffee-script` and `coffee-script-source` gems.
  #
  # For more infomation see:
  #
  #   https://github.com/josh/ruby-coffee-script
  #
  module CoffeeScriptProcessor
    VERSION = '1'

    def self.cache_key
      @cache_key ||= "#{name}:#{Autoload::CoffeeScript::Source.version}:#{VERSION}".freeze
    end

    # Returning a string here is short-hand for `{data: <string>}`; other return hash keys are possible.
    #
    # Helpful input keys:
    #   input[:filename] # => CoffeeScript filename (could be ERB, then CoffeeScript)
    #   input[:name]     # => CoffeeScript asset (looks like :filename without suffix)
    #   input[:data]     # => CoffeeScript source (after other processing; possibly ERB, etc.)

    def self.call(input)
      data = input[:data]
      compiled = input[:cache].fetch([self.cache_key, data]) do
        Autoload::CoffeeScript.compile(data, { sourceMap: true }) # Hash key must be cased as expected by CoffeeScript
      end

      # Investigate `compiled`, it contains several versions of source maps.

      # Only write a sourcemap if the JavaScript file has content remaining
      # after Sprockets processing (i.e., require directives, etc. have been
      # removed.  Sprockets leaves behind blank lines after removing directives,
      # therefore strip the processed output.
      if ! data.strip.empty?
        sourcemap = JSON.parse(compiled["v3SourceMap"]) # => {"version"=>3, "file"=>"", "sourceRoot"=>"", "sources"=>[""], "names"=>[], "mappings"=>"AAAA;;AAAA"}

        # Investigate values for "names", "file", "sourceRoot".
        sourcemap["sources"] = [input[:name] + '.coffee']
        sourcemap["sourcesContent"] = [data]       # Optional: not needed if the `.coffee` files are available on the server; but placeholder nil required?
        # sourcemap["sourceRoot"] = ["sourcemaps"] # Optional: server path to sourcemaps root, if location is different
        # sourcemap["names"] = []                  # Symbol names used by the mappings entry.
        sourcemap_json = sourcemap.to_json

        # Write the sourcemap to the Blade build path.
        sourcemap_filename = "#{input[:name]}-#{digest(sourcemap_json)}.coffee.map"
        sourcemap_path = File.join(Blade.config.build.path, sourcemap_filename)
        FileUtils.mkdir_p File.dirname(sourcemap_path)
        File.open(sourcemap_path, 'w') { |f| f.write sourcemap_json }

        compiled["js"].concat "\n//# sourceMappingURL=#{sourcemap_filename}\n"
      else
        compiled["js"] # => JS after CS compilation
      end
    end

    def self.digest(io)
      Sprockets::DigestUtils.pack_hexdigest Sprockets::DigestUtils.digest(io)
    end
  end
end

simple.coffee

(-> throw Error)()