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Troubleshooting

Flash of Unstyled Content (FOUC)​

If you're experiencing FOUC where content briefly appears unstyled before CSS loads, see the Preventing FOUC guide for solutions including proper stylesheet_pack_tag placement and content_for patterns for dynamic pack loading.

Debugging your webpack config​

  1. Read the error message carefully. The error message will tell you the precise key value that is not matching what Webpack expects.

  2. Put a debugger statement in your Webpack configuration and run bin/shakapacker --debug-shakapacker. If you have a node debugger installed, you'll see the Chrome debugger for your webpack config. For example, install the Chrome extension NiM and set the option for the dev tools to open automatically. Or put chrome://inspect in the URL bar. For more details on debugging, see the official Webpack docs on debugging

  3. Any arguments that you add to bin/shakapacker get sent to webpack. For example, you can pass --debug to switch loaders to debug mode. See webpack CLI debug options for more information on the available options.

  4. You can also pass additional options to the command to run the webpack-dev-server and start the webpack-dev-server with the option --debug-shakapacker

  5. Export your full webpack/rspack configuration for analysis: Use the bin/shakapacker-config utility to export your complete resolved configuration. This is especially helpful for:

    • Migrations: Comparing configurations before and after migrating between webpack and rspack, or between different Shakapacker versions
    • Debugging: Inspecting the exact configuration webpack/rspack is using, including all merged settings
    • AI Analysis: Uploading the exported config to ChatGPT or other AI tools for troubleshooting

    Installation: The utility is installed when you run rake shakapacker:binstubs or can be used directly via rake shakapacker:export_bundler_config.

    RECOMMENDED - Quick troubleshooting:

    # Install the binstub (one-time setup)
    rake shakapacker:binstubs

    # Export EVERYTHING for troubleshooting (dev + prod, annotated YAML)
    bin/shakapacker-config --doctor
    # Creates: webpack-development-client.yaml, webpack-development-server.yaml,
    #          webpack-production-client.yaml, webpack-production-server.yaml

    Other usage examples:

    # Save current environment configs with auto-generated names
    bin/shakapacker-config --save
    # Creates: webpack-development-client.yaml, webpack-development-server.yaml

    # Save to specific directory
    bin/shakapacker-config --save --save-dir=./debug-configs

    # Export only client config for production
    bin/shakapacker-config --save --env=production --client-only
    # Creates: webpack-production-client.yaml

    # Compare development vs production configs
    bin/shakapacker-config --save --save-dir=./configs
    diff configs/webpack-development-client.yaml configs/webpack-production-client.yaml

    # View config in terminal (no files created)
    bin/shakapacker-config

    # Export without inline documentation annotations
    bin/shakapacker-config --save --no-annotate

    # Export in JSON format for programmatic analysis
    bin/shakapacker-config --save --format=json

    Config files are automatically named: {bundler}-{env}-{type}.{ext}

    • Examples: webpack-development-client.yaml, rspack-production-server.yaml
    • YAML format includes inline documentation explaining each config key
    • Separate files for client and server bundles (cleaner than combined)

    See bin/shakapacker-config --help for all available options.

  6. Validate your webpack/rspack builds: Use bin/shakapacker-config --validate to test that all your build configurations compile successfully. This is especially useful for:

    • CI/CD pipelines: Catch configuration errors before deployment
    • Migration testing: Verify builds work after upgrading webpack, rspack, or Shakapacker
    • Multi-environment testing: Ensure all build configurations (dev, prod, HMR) compile correctly

    Quick validation:

    # Validate all builds defined in .bundler-config.yml
    bin/shakapacker-config --validate

    # Validate with full output logs (shows all webpack/rspack compilation output)
    bin/shakapacker-config --validate --verbose

    # Validate a specific build
    bin/shakapacker-config --validate-build=dev-hmr

    Verbose Mode:

    When using --verbose, you'll see:

    • A clear header indicating verbose mode is enabled
    • Full real-time compilation output from webpack/rspack
    • All warnings and progress messages
    • Detailed error traces
    • Separators between builds for clarity

    This is useful for debugging compilation issues or understanding build performance.

    Setting up build configurations:

    # Create a .bundler-config.yml file with example builds
    bin/shakapacker-config --init

    # List all available builds
    bin/shakapacker-config --list-builds

    # Validate all builds
    bin/shakapacker-config --validate

    Advanced options:

    The validator uses a default timeout of 2 minutes per build. For large projects or slow CI environments, you can customize this behavior by modifying the ValidatorOptions in your code, or by adjusting your build configuration to be more efficient.

    If validation times out, try:

    • Using --verbose to see where the build is hanging
    • Optimizing your webpack/rspack configuration for faster builds
    • Running validation on a single build with --validate-build=build-name

    How it works:

    The validator will:

    • For HMR builds (with WEBPACK_SERVE=true): Start webpack-dev-server, wait for successful compilation, then shut down
    • For static builds: Run webpack/rspack and check for compilation errors
    • Report all errors and warnings with clear output
    • Exit with code 1 if any build fails (perfect for CI)

    Example output:

    🔍 Validating Builds
    ================================================================================

    📦 Validating build: dev-hmr
       ✅ Build passed

    📦 Validating build: dev
       ✅ Build passed

    📦 Validating build: prod
       ❌ Build failed with 2 error(s)

    ================================================================================
    🔍 Build Validation Results
    ================================================================================

    ✅ Build: dev-hmr (2.34s)
       📦 Outputs: client
       ⚙️  Config: config/webpack/webpack.config.js

    ✅ Build: dev (3.12s)
       📦 Outputs: client, server
       ⚙️  Config: config/webpack/webpack.config.js
       📁 Output: /app/public/packs

    ❌ Build: prod (4.56s)
       📦 Outputs: client, server
       ⚙️  Config: config/webpack/webpack.config.js
       📁 Output: /app/public/packs
       ❌ 2 error(s)
          Module not found: Error: Can't resolve './missing'
          SyntaxError: Unexpected token

    ================================================================================
    Summary: 2/3 builds passed, 1 failed (Total: 10.02s)
    ================================================================================

    💡 Debugging Tips:
       To get more details, run individual builds with --verbose:

       bin/shakapacker-config --validate-build prod --verbose

       Or validate all builds with full output: bin/shakapacker-config --validate --verbose
    ================================================================================

    Debugging Failed Builds:

    When builds fail, the validator automatically provides debugging commands. You can:

    1. Run a specific build with verbose output to see full webpack/rspack logs:

      bin/shakapacker-config --validate-build prod --verbose

    2. Validate all builds with verbose output to see everything:

      bin/shakapacker-config --validate --verbose

    3. Test individual builds manually using the same configuration:

      # For static builds
      NODE_ENV=production RAILS_ENV=production bundle exec webpack --config config/webpack/webpack.config.js

      # For HMR/dev-server builds
      NODE_ENV=development WEBPACK_SERVE=true bundle exec webpack serve --config config/webpack/webpack.config.js

    The verbose mode shows:

    • Full real-time compilation output
    • All webpack/rspack warnings and progress messages
    • Detailed stack traces for errors
    • Timing information for each build phase
    • Clear separators between different builds

  7. Generate webpack stats for build analysis (useful for bundle size optimization):

    NODE_ENV=development bin/shakapacker --profile --json > /tmp/webpack-stats.json

    ChatGPT and other AI tools can consume this output file. Change the NODE_ENV per your needs.

Incorrect peer dependencies​

Shakapacker uses peer dependencies to make it easier to manage what versions are being used for your app, which is especially useful for patching security vulnerabilities. However, not all package managers will actually enforce these versions - notably, Yarn will omit a warning rather than erroring if you forget to update a peer dependency:

warning " > shakapacker@6.1.1" has incorrect peer dependency "compression-webpack-plugin@^9.0.0".

This omission resulted in an error in the browser:

Failed to load resource: net::ERR_CONTENT_DECODING_FAILED

The error was caused by an old version of the peer dependency webpack-compression-plugin.

So, be sure to investigate warnings from yarn install!

NoMethodError: undefined method 'deep_symbolize_keys' for nil:NilClass​

If you see this error during deployment (especially on Heroku with a staging environment):

NoMethodError: undefined method 'deep_symbolize_keys' for nil:NilClass
  from shakapacker/configuration.rb:XXX:in 'load'

This happens when deploying to a custom Rails environment (like staging) that isn't explicitly defined in your config/shakapacker.yml file.

Solution: This was fixed in Shakapacker v9.1.1+. Upgrade to the latest version:

# Gemfile
gem 'shakapacker', '~> 9.1'

After upgrading, Shakapacker will automatically fall back to sensible defaults when your environment isn't defined:

  1. First tries your environment (e.g., staging)
  2. Falls back to production configuration

Alternative: If you can't upgrade immediately, explicitly add your environment to config/shakapacker.yml:

staging:
  <<: *default
  compile: false
  cache_manifest: true

See the deployment guide for more details.

ENOENT: no such file or directory - node-sass​

If you get the error ENOENT: no such file or directory - node-sass on deploy with assets:precompile or bundle exec rake shakapacker:compile you may need to move Sass to production dependencies.

Move any packages that related to Sass (e.g. node-sass or sass-loader) from devDependencies to dependencies in package.json. This is because shakapacker is running on a production system with the Rails workflow to build the assets. Particularly on hosting providers that try to detect and do the right thing, like Heroku.

However, if you get this on local development, or not during a deploy then you may need to rebuild node-sass. It's a bit of a weird error; basically, it can't find the node-sass binary. An easy solution is to create a postinstall hook to ensure node-sass is rebuilt whenever new modules are installed.

In package.json:

"scripts": {
  "postinstall": "npm rebuild node-sass"
}

Can't find hello_react.js in manifest.json​

  • If you get this error Can't find hello_react.js in manifest.json when loading a view in the browser it's because webpack is still compiling packs. Shakapacker uses a manifest.json file to keep track of packs in all environments, however since this file is generated after packs are compiled by webpack. So, if you load a view in browser whilst webpack is compiling you will get this error. Therefore, make sure webpack (i.e ./bin/shakapacker-dev-server) is running and has completed the compilation successfully before loading a view.

throw er; // Unhandled 'error' event​

  • If you get this error while trying to use Elm, try rebuilding Elm. You can do so with a postinstall hook in your package.json:
"scripts": {
  "postinstall": "npm rebuild elm"
}

webpack or webpack-dev-server not found​

  • This could happen if shakapacker:install step is skipped. Please run bundle exec rake shakapacker:install to fix the issue.

  • If you encounter the above error on heroku after upgrading from Rails 4.x to 5.1.x, then the problem might be related to missing yarn binstub. Please run following commands to update/add binstubs:

bundle config --delete bin
bundle exec rake app:update:bin

Running webpack on Windows​

If you are running webpack on Windows, your command shell may not be able to interpret the preferred interpreter for the scripts generated in bin/shakapacker and bin/shakapacker-dev-server. Instead you'll want to run the scripts manually with Ruby:

C:\path>ruby bin\webpack
C:\path>ruby bin\webpack-dev-server

Invalid configuration object. webpack has been initialised using a configuration object that does not match the API schema.​

If you receive this error when running $ ./bin/shakapacker-dev-server ensure your configuration is correct; most likely the path to your "packs" folder is incorrect if you modified from the original "source_path" defined in config/shakapacker.yml.

Running Elm on Continuous Integration (CI) services such as CircleCI, CodeShip, Travis CI​

If your tests are timing out or erroring on CI it is likely that you are experiencing the slow Elm compilation issue described here: elm-compiler issue #1473

The issue is related to CPU count exposed by the underlying service. The basic solution involves using libsysconfcpus to change the reported CPU count.

Basic fix involves:

# install sysconfcpus on CI

git clone https://github.com/obmarg/libsysconfcpus.git $HOME/dependencies/libsysconfcpus
cd libsysconfcpus
.configure --prefix=$HOME/dependencies/sysconfcpus
make && make install

# use sysconfcpus with elm-make
mv $HOME/your_rails_app/node_modules/.bin/elm-make $HOME/your_rails_app/node_modules/.bin/elm-make-old
printf "#\041/bin/bash\n\necho \"Running elm-make with sysconfcpus -n 2\"\n\n$HOME/dependencies/sysconfcpus/bin/sysconfcpus -n 2 $HOME/your_rails_app/node_modules/.bin/elm-make-old \"\$@\"" > $HOME/your_rails_app/node_modules/.bin/elm-make
chmod +x $HOME/your_rails_app/node_modules/.bin/elm-make

Rake assets:precompile fails. ExecJS::RuntimeError​

This error occurs because you are trying to minify by terser a pack that's already been minified by Shakapacker. To avoid this conflict and prevent appearing of ExecJS::RuntimeError error, you will need to disable uglifier from Rails config:

# In production.rb

# From
Rails.application.config.assets.js_compressor = :uglifier

# To
Rails.application.config.assets.js_compressor = Uglifier.new(harmony: true)

Angular: WARNING in ./node_modules/@angular/core/esm5/core.js, Critical dependency: the request of a dependency is an expression​

To silent these warnings, please update config/webpack/webpack.config.js:

const webpack = require("webpack")
const { resolve } = require("path")
const { generateWebpackConfig } = require("shakapacker")

module.exports = generateWebpackConfig({
  plugins: [
    new webpack.ContextReplacementPlugin(
      /angular(\\|\/)core(\\|\/)(@angular|esm5)/,
      resolve(config.source_path)
    )
  ]
})

Compilation Fails Silently​

If compiling is not producing output files and there are no error messages to help troubleshoot. Setting the webpack_compile_output configuration variable to true in shakapacker.yml may add some helpful error information to your log file (Rails log/development.log or log/production.log)

# shakapacker.yml
default: &default
  source_path: app/javascript
  source_entry_path: packs
  public_root_path: public
  public_output_path: complaints_packs
  webpack_compile_output: true

Using global variables for dependencies​

If you want to access any dependency without importing it everywhere or use it directly in your dev tools, please check: https://webpack.js.org/plugins/provide-plugin/ and the webpack docs on shimming globals.

Note, if you are exposing globals, like jQuery, to non-webpack dependencies (like an inline script) via the expose-loader, you will need to override the default of defer: true to be defer:false your call to the javascript_pack_tag so that the browser will load your bundle to set up the global variable before other code depends on it. However, you really should try to remove the dependency on such globals.

Thus ProvidePlugin manages build-time dependencies to global symbols whereas the expose-loader manages runtime dependencies to global symbols.

You don't need to assign dependencies on window.

For instance, with jQuery:

// app/javascript/entrypoints/application.js

- import jQuery from 'jquery'
- window.jQuery = jQuery

Instead do:

// config/webpack/webpack.config.js

const webpack = require("webpack")
const { generateWebpackConfig } = require("shakapacker")

module.exports = generateWebpackConfig({
  plugins: [
    new webpack.ProvidePlugin({
      $: "jquery",
      jQuery: "jquery"
    })
  ]
})

Wrong CDN src from javascript_pack_tag​

If your deployment doesn't rebuild assets between environments (such as when using Heroku's Pipeline promote feature). You might find that your production application is using your staging config.asset_host host when using javascript_pack_tag.

This can be fixed by setting the environment variable SHAKAPACKER_ASSET_HOST to an empty string where your assets are compiled. On Heroku this is done under Settings -> Config Vars.

This way shakapacker won't hard-code the CDN host into the manifest file used by javascript_pack_tag, but instead fetch the CDN host at runtime, resolving the issue.

See this issue for more details.

Static file dependencies emitted outside of public output path​

For static file assets (images, fonts), we use a Webpack rule to handle those files as asset/resource type and output them in the static folder in the public output path.

In order to generate the storage path, we rely on the filename that's provided by webpack internals.

This usually works out of the box. There's a potential problem however, if you use the context setting in your webpack config. By default this is set to current Node working directory/project root.

If you were to override it like:

{
  context: path.resolve(__dirname, '../../app/javascript')
}

Then the filename available in the rule generator will be relative to that directory.

This means for example:

  • a static asset from node_modules folder could end up being referenced with path of ../../node_modules/some_module/static_file.jpg rather than simply node_modules/some_module/static_file.jpg.
  • a static asset in one of the additional_paths, example app/assets/images/image.jpg, would end up being referenced with path of ../assets/images/image.jpg.

Those paths are later passed to output path generation in the rule, where we would end up with a path like static/../../node_modules/some_module/static_file.jpg, resulting in the file being output in a location two directories above the desired path.

You can avoid this by:

  • not using overridden context in your webpack config, if there's no good reason for it.
  • using custom Webpack config to modify the static file rule, following a similar process as outlined in the Webpack Configuration section of the readme.

See this issue for more details.