Instrument JavaScript code with Istanbul when running system tests on Rails 7 with importmaps

application_system_test_case.rb

# test/application_system_test_case.rb

require "test_helper"

class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
  include IstanbulInstrumentation

  instrument_dir "app/javascript/controllers"

  driven_by :selenium, using: :chrome, screen_size: [1400, 1400]
end

importmap.rb

# config/importmap.rb

pin "application"
pin "@hotwired/turbo-rails", to: "turbo.min.js"
pin "@hotwired/stimulus", to: "stimulus.min.js"
pin "@hotwired/stimulus-loading", to: "stimulus-loading.js"

if Rails.env.test?
  # Use instrumented code to collect coverage.
  pin_all_from "app/javascript/controllers-instrumented", under: "controllers", to: "controllers-instrumented"
else
  pin_all_from "app/javascript/controllers", under: "controllers"
end

istanbul_instrumentation.rb

# lib/istanbul_instrumentation.rb

# Mixin enabling Istanbul instrumentation and generating coverage report.
# It uses `npx` to download and run `nyc`.
module IstanbulInstrumentation
  extend ActiveSupport::Concern

  COVERAGE_DIR = Rails.root.join(".nyc_output")

  class_methods do
    # Add path to the list of directories to be instrumented.
    def instrument_dir(dirname)
      directories << instrument_code(dirname)
    end

    # Runs a command in subshell. Throws exception on non-zero exit codes or errors.
    def run_command(cmd)
      system cmd, exception: true
    end

    # Runs istanbul client to instrument the JavaScript code.
    # Returns the instrumented code path.
    def instrument_code(source)
      destination = "#{source}-instrumented"
      puts "Instrumenting the code at #{source} into #{destination}"
      run_command "npx nyc instrument #{source} #{destination}"

      destination
    end

    # Generates istanbul reports in the given format (default: text).
    def generate_coverage_report(*formats)
      puts "Generating Istanbul coverage report"
      run_command "npx nyc report #{formats.map { |fmt| "--reporter=#{fmt}" }.join " "}"
    end

    # Removes instrumented code and intermediate coverage reports.
    def cleanup
      FileUtils.rm_r [ COVERAGE_DIR, *directories ]
    end
  end

  included do
    class_attribute :directories, default: []

    Minitest.after_run do
      generate_coverage_report "lcov", "text-summary"
      cleanup
    end
  end

  def teardown
    super
    collect_coverage
  end

  # Collect the coverage from the instrumented code and save into JSON file.
  def collect_coverage
    coverage = page.evaluate_script("window.__coverage__")
    return unless coverage.present?

    FileUtils.mkdir COVERAGE_DIR unless Dir.exist? COVERAGE_DIR
    filename = File.join(COVERAGE_DIR, "coverage_#{self.class_name.tableize}_#{self.name}.json")
    File.open(filename, "w") do |file|
      file.puts JSON.pretty_generate coverage
    end
  end
end