Custom sprockets compressor as proxy skips minified assets

Readme.md

Proposal

Many modern Rails application use third-party assets like JavaScript and CSS libraries or frameworks. In most cases these assets are minified and compressed by tools outside of the Rails asset pipeline. For example popular gem React on Rails uses WebPack to build JavaScript bundles. When we include these files to assets pipeline it goes to minify again. We can safely avoid double minification and reduce assets compilation time. Meanwhile it is still good to add a digest and gzip compression.

Example

Savings are strongly depend on how many minified assets are used in an application, some numbers for a random application:

# standard setup

$ time RAILS_ENV=production rake assets:precompile

real    2m44.868s
user    2m25.503s
sys    0m17.170s

# avoid double minification 

$ time RAILS_ENV=production rake assets:precompile

real    2m2.841s
user    1m53.312s
sys    0m10.562s

application.rb

require File.expand_path("../../lib/skippable_compressor", __FILE__)

module MyApp
  class Application < Rails::Application
    # Register and use SkippableCompressor for css and javascript files
    Sprockets.register_compressor "text/css", :skippable_compressor, SkippableCompressor
    Sprockets.register_compressor "application/javascript", :skippable_compressor, SkippableCompressor
    config.assets.css_compressor = :skippable_compressor
    config.assets.js_compressor = :skippable_compressor
    
    # include minified bundles to pipeline
    config.assets.precompile += %w[
      generated/app1-bundle.min.js
      generated/app2-bundle.min.js
      generated/app3-bundle.min.js
    ]
  end
end

skippable_compressor.rb

# lib/skippable_compressor.rb

# Compressor class, skips files based on filename pattern
# Uses default compressors for CSS and JavaScript files, if assets are not minified
#
# Skips compression for filename 'jquery.min.js' but not for 'jquery.js'
# Skips compression for filename 'bootstrap.min.css' but not for 'bootstrap.css'
#
class SkippableCompressor
  FILENAME_PATTERNS = {
    "application/javascript" => /\.min\.js/,
    "text/css" => /\.min\.css/,
  }.freeze

  def self.default_css_compressor
    @default_css_compressor ||= Sprockets::SassCompressor.instance
  end

  def self.default_js_compressor
    @default_js_compressor ||= Sprockets::UglifierCompressor.instance
  end

  def self.default_compressors
    @default_compressors ||= {
      "application/javascript" => default_js_compressor,
      "text/css" => default_css_compressor,
    }
  end

  def self.call(input)
    content_type = input[:content_type]

    if input[:filename] =~ FILENAME_PATTERNS[content_type]
      input[:data]
    else
      default_compressors[content_type].call(input)
    end
  end
end

webpack.client.rails.config.js

// Example for WebPack config
// add min suffix for generated files with React applications
config.output = {
  filename: '[name]-bundle.min.js',
  path: '../app/assets/javascripts/generated',
};