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Here is the full set of config options. This file is /config/initializers/react_on_rails.rb

First, you should have a /config/webpacker.yml setup.

Here is the setup when using the recommended /client directory for your node_modules and source files:

# Note: Base output directory of /public is assumed for static files
default: &default
  compile: false
  # Used in your webpack configuration. Must be created in the
  # public_output_path folder
  manifest: manifest.json
  cache_manifest: false

  # Source path is used to check if webpack compilation needs to be run for `compile: true`
  source_path: client/app

development:
  <<: *default
  # Generated files for development, in /public/webpack/dev
  public_output_path: webpack/dev

test:
  <<: *default
  # Ensure that webpacker invokes webpack to build files for tests if not using the
  #   ReactOnRails rspec helper.
  compile: true

  # Generated files for tests, in /public/webpack/test
  public_output_path: webpack/test

production:
  <<: *default
  # Generated files for production, in /public/webpack/production
  public_output_path: webpack/production
  cache_manifest: true

Here's a representative /config/initializers/react_on_rails.rb setup when using this /client directory for all client files, including your sources and node_modules.

# frozen_string_literal: true

# NOTE: you typically will leave the commented out configurations set to their defaults.
# Thus, you only need to pay careful attention to the non-commented settings in this file.
ReactOnRails.configure do |config|
  # `trace`: General debugging flag.
  # The default is true for development, off otherwise.
  # With true, you get detailed logs of rendering and stack traces if you call setTimout,
  # setInterval, clearTimout when server rendering.
  config.trace = Rails.env.development? # default

  # Configure if default DOM IDs have a random value or are fixed.
  # false ==> Sets the dom id to "#{react_component_name}-react-component"
  # true ==> Adds "-#{SecureRandom.uuid}" to that ID
  # If you might use multiple instances of the same React component on a Rails page, then
  # it is convenient to set this to true or else you have to either manually set the ids to
  # avoid collisions. Most newer apps will have only one instance of a component on a page,
  # so this should be false in most cases.
  # This value can be overridden for a given call to react_component
  config.random_dom_id = true # default

  # defaults to "" (top level)
  config.node_modules_location = "client" # If using webpacker you should use "".

  # This configures the script to run to build the production assets by webpack . Set this to nil
  # if you don't want react_on_rails building this file for you.
  # Note, if you want to use this command then you should remove the file
  # config/webpack/production.js
  # If that file exists, React on Rails thinks that you'll use the rails/webpacker bin/webpack compiler.
  config.build_production_command = "RAILS_ENV=production bin/webpack"

  # Alternatively, you can also specify a module containing a class method `call`
  # In this example, the module BuildProductionCommand would have a class method `call`.
  # See bottom for an example of the BuildProductionCommand module.
  # config.build_production_command = BuildProductionCommand
  # If you wish to utilize ReactOnRailsPro production bundle caching logic, then use
  # config.build_production_command = ReactOnRailsPro::AssetsPrecompile
  # and be sure to check ReactOnRailsPro's configuration documentation!

  ################################################################################
  ################################################################################
  # SERVER RENDERING OPTIONS
  ################################################################################
  # This is the file used for server rendering of React when using `(prerender: true)`
  # If you are never using server rendering, you should set this to "".
  # Note, there is only one server bundle, unlike JavaScript where you want to minimize the size
  # of the JS sent to the client. For the server rendering, React on Rails creates a pool of
  # JavaScript execution instances which should handle any component requested.
  #
  # While you may configure this to be the same as your client bundle file, this file is typically
  # different. Note, be sure to include the exact file name with the ".js" if you are not hashing this file.
  # If you are hashing this file (supposing you are using the same file for client rendering), then
  # you should include a name that matches your bundle name in your webpack config.
  config.server_bundle_js_file = "server-bundle.js"

  # THE BELOW OPTIONS FOR SERVER-SIDE RENDERING RARELY NEED CHANGING
  #
  # This value only affects server-side rendering when using the webpack-dev-server
  # If you are hashing the server bundle and you want to use the same bundle for client and server,
  # you'd set this to `true` so that React on Rails reads the server bundle from the webpack-dev-server.
  # Normally, you have different bundles for client and server, thus, the default is false.
  # Furthermore, if you are not hashing the server bundle (not in the manifest.json), then React on Rails
  # will only look for the server bundle to be created in the typical file location, typically by
  # a `webpack --watch` process.
  # If true, ensure that in config/webpacker.yml that you have both dev_server.hmr and
  # dev_server.inline set to false.
  config.same_bundle_for_client_and_server = false

  # If set to true, this forces Rails to reload the server bundle if it is modified
  # Default value is Rails.env.development?
  # You probably will never change this.
  config.development_mode = Rails.env.development?

  # For server rendering so that the server-side console replays in the browser console.
  # This can be set to false so that server side messages are not displayed in the browser.
  # Default is true. Be cautious about turning this off, as it can make debugging difficult.
  # Default value is true
  config.replay_console = true

  # Default is true. Logs server rendering messages to Rails.logger.info. If false, you'll only
  # see the server rendering messages in the browser console.
  config.logging_on_server = true

  # Default is true only for development? to raise exception on server if the JS code throws for
  # server rendering. The reason is that the server logs will show the error and force you to fix
  # any server rendering issues immediately during development.
  config.raise_on_prerender_error = Rails.env.development?

  ################################################################################
  # Server Renderer Configuration for ExecJS
  ################################################################################
  # The default server rendering is ExecJS, probably using the mini_racer gem
  # If you wish to use an alternative Node server rendering for higher performance,
  # contact justin@shakacode.com for details.
  #
  # For ExecJS:
  # You can configure your pool of JS virtual machines and specify where it should load code:
  # On MRI, use `mini_racer` for the best performance
  # (see [discussion](https://github.com/reactjs/react-rails/pull/290))
  # On MRI, you'll get a deadlock with `pool_size` > 1
  # If you're using JRuby, you can increase `pool_size` to have real multi-threaded rendering.
  config.server_renderer_pool_size = 1 # increase if you're on JRuby
  config.server_renderer_timeout = 20 # seconds

  ################################################################################
  # I18N OPTIONS
  ################################################################################
  # Replace the following line to the location where you keep translation.js & default.js for use
  # by the npm packages react-intl. Be sure this directory exists!
  # config.i18n_dir = Rails.root.join("client", "app", "libs", "i18n")
  #
  # If not using the i18n feature, then leave this section commented out or set the value
  # of config.i18n_dir to nil.
  #
  # Replace the following line to the location where you keep your client i18n yml files
  # that will source for automatic generation on translations.js & default.js
  # By default(without this option) all yaml files from Rails.root.join("config", "locales")
  # and installed gems are loaded
  config.i18n_yml_dir = Rails.root.join("config", "locales")

  # Possible output formats are js and json
  # The default format is json
  config.i18n_output_format = 'json'

  ################################################################################
  ################################################################################
  # CLIENT RENDERING OPTIONS
  # Below options can be overriden by passing options to the react_on_rails
  # `render_component` view helper method.
  ################################################################################
  # default is false
  config.prerender = false

  # You can optionally add values to your rails_context. This object is passed
  # every time a component renders.
  # See example below for an example definition of RenderingExtension
  #
  # config.rendering_extension = RenderingExtension

  ################################################################################
  ################################################################################
  # TEST CONFIGURATION OPTIONS
  # Below options are used with the use of this test helper:
  # ReactOnRails::TestHelper.configure_rspec_to_compile_assets(config)
  #
  # NOTE:
  # Instead of using this test helper, you may ensure fresh test files using rails/webpacker via:
  # 1. Have `config/webpacker/test.js` exporting an array of objects to configure both client and server bundles.
  # 2. Set the compile option to true in config/webpacker.yml for env test
  ################################################################################

  # If you are using this in your spec_helper.rb (or rails_helper.rb):
  #
  # ReactOnRails::TestHelper.configure_rspec_to_compile_assets(config)
  #
  # with rspec then this controls what yarn command is run
  # to automatically refresh your webpack assets on every test run.
  #
  config.build_test_command = "RAILS_ENV=test bin/webpack"

  # CONFIGURE YOUR SOURCE FILES
  # The test helper needs to know where your JavaScript files exist. The value is configured
  # by your config/webpacker.yml source_path:
  # source_path: client/app # if using recommended /client directory
  #
  # Define the files we need to check for webpack compilation when running tests.
  # The default is `%w( manifest.json )` as will be sufficient for most webpacker builds.
  # However, if you are generated a server bundle that is NOT hashed (present in manifest.json),
  # then include the file in this list like this:
  config.webpack_generated_files = %w( server-bundle.js manifest.json )
  # Note, be sure NOT to include your server-bundle.js if it is hashed, or else React on Rails will
  # think the server-bundle.js is missing every time for test runs.
end

Example of a ReactOnRailsConfig module for production_build_command:

module BuildProductionCommand
  include FileUtils
  # Method with the name of call will be called during assets:precompile
  def self.call
    sh "bin/webpack"
  end
end

Example of a RenderingExtension for custom values in the rails_context:

module RenderingExtension

  # Return a Hash that contains custom values from the view context that will get merged with
  # the standard rails_context values and passed to all calls to Render-Functions used by the
  # react_component and redux_store view helpers
  def self.custom_context(view_context)
    {
     somethingUseful: view_context.session[:something_useful]
    }
  end
end