Add source map for sprockets

app/assets/config/manifest.js

@@ -1,3 +1,5 @@
 //= link_tree ../images
 //= link_directory ../javascripts .js
+//= link_directory ../javascripts .js.map
 //= link_directory ../stylesheets .css
+//= link_directory ../stylesheets .css.map

lib/tasks/sourcemap.rake

@@ -0,0 +1,87 @@
+# frozen_string_literal: true
+
+# Adapted from https://github.com/rails/sprockets/issues/502#issuecomment-1030634236
+
+SOURCE_EXTENSIONS = %w[.css .js].freeze
+
+namespace :assets do
+  def append_sourcemap
+    manifest_path = Dir[Rails.public_path.join('assets/.sprockets-manifest-*.json').to_s].max {|a, b| File.ctime(a) <=> File.ctime(b) }
+    return unless manifest_path
+
+    manifest_json = JSON.parse(File.read(manifest_path))
+    assets = manifest_json['assets']
+    manifest = manifest_json['files']
+
+    assets.each do |name, digested|
+      ext = File.extname(name)
+      next unless SOURCE_EXTENSIONS.include? ext
+
+      # Get the source map file from the manifest
+      map_digested = assets["#{name}.map"]
+      next unless map_digested
+
+      # Parse the source map file
+      source_map = JSON.parse(File.read(Rails.public_path.join('assets', map_digested).to_s))
+
+      # Replace the source map file name with the digested one
+      if source_map['sections'].present?
+        source_map['sections'].map! do |section|
+          section['map']['sources'].map! {|source| assets[source] || source }
+          section
+        end
+      elsif source_map['sources'].present?
+        source_map['sources'].map! {|source| assets[source] || source }
+      end
+
+      # Write the source map file
+      asset_source = Rails.public_path.join('assets', map_digested).to_s
+      write_asset(asset_source, source_map.to_json, manifest)
+
+      # Construct the source map link
+      file = Rails.root.join("public/assets/#{digested}")
+      mapping_string = "sourceMappingURL=#{map_digested}"
+
+      mapping_string = case ext
+                         when '.css' then "/*# #{mapping_string} */"
+                         when '.js' then "//# #{mapping_string}"
+                       end
+
+      # Read the source map file and append the source map link
+      write_asset(file, "#{file.read}\n#{mapping_string}", manifest)
+    end
+
+    File.write(manifest_path, manifest_json.to_json)
+  end
+
+  def write_asset(filename, content, manifest)
+    File.write(filename, content)
+    mtime = Sprockets::PathUtils.stat(filename)&.mtime
+    compress_asset(filename, content, mtime)
+    manifest[File.basename(filename)].merge!(changed_manifest(content, mtime))
+  end
+
+  def compress_asset(source_filename, content, mtime)
+    target = "#{source_filename}.gz"
+
+    File.open(target, 'wb') do |file|
+      Sprockets::Utils::Gzip::ZlibArchiver.call(file, content, mtime)
+    end
+  end
+
+  def changed_manifest(content, mtime)
+    digest = Sprockets::DigestUtils.digest(content)
+    hexdigest = Sprockets::DigestUtils.pack_hexdigest(digest)
+
+    {
+      'mtime' => mtime,
+      'size' => content.bytesize,
+      'digest' => hexdigest,
+      'integrity' => Sprockets::DigestUtils.hexdigest_integrity_uri(hexdigest),
+    }
+  end
+
+  task precompile: :environment do
+    append_sourcemap
+  end
+end