Troubleshooting

Debugging your webpack config

Read the error message carefully. The error message will tell you the precise key value that is not matching what Webpack expects.
Put a debugger statement in your Webpack configuration and run bin/webpacker --debug-webpacker. 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
Any arguments that you add to bin/webpacker 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.
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-webpacker

Incorrect peer dependencies

The latest version of Shakapacker uses peer dependencies to make upgrading easier. However, there's a catch.

If you fail to update peer dependencies, you'll see an easy-to-overlook warning from yarn install.

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!

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 rails webpacker: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 webpacker 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. Webpacker 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/webpacker-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 webpacker:install step is skipped. Please run bundle exec rails webpacker: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
./bin/rails app:update:bin # or rails 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/webpacker and bin/webpacker-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/webpacker-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/webpacker.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 Webpacker. 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 { webpackConfig, merge } = require('shakapacker')

module.exports = merge(webpackConfig, {
  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 webpacker.yml may add some helpful error information to your log file (Rails log/development.log or log/production.log)

# webpacker.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 setup the global variable before other code depends on it. However, you really should try to remove the dependendency 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/packs/entrypoints/application.js

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

Instead do:

// config/webpack/webpack.config.js

const webpack = require('webpack')
const { webpackConfig, merge } = require('shakapacker')

module.exports = merge(webpackConfig, {
  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 WEBPACKER_ASSET_HOST to an empty string where your assets are compiled. On Heroku this is done under Settings -> Config Vars.

This way webpacker 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.

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