Rails: Webpacker 4.2 can't find application in /app/public/packs/manifest.json heroku

Question

I'm a bit stumped. My local rails app works great with webpacker 4.2 and react, but when deploying to production gives me the wonderful can't find application in /app/public/packs/manifest.json error.

Here's what I've tried:

• Tried adding/removing turbo link details for javascript pack tag.
• Removed javascript include tag (not sure if this helps or not)
• Ensure I'm on the latest webpacker 4.2
• ran rake assets:clean && rake assets:precompile manually on heroku just to ensure things are getting built.

Am I missing a build step or something in production that would be causing this? What is left to check?

Server error:

2019-12-03T15:18:19.022024+00:00 app[web.1]: I, [2019-12-03T15:18:19.021952 #30]  INFO -- : [aa0374eb-bab1-40cc-b40b-6ae3d195e07d] Completed 500 Internal Server Error in 112ms (ActiveRecord: 30.4ms | Allocations: 21296)
2019-12-03T15:18:19.023103+00:00 app[web.1]: F, [2019-12-03T15:18:19.023029 #30] FATAL -- : [aa0374eb-bab1-40cc-b40b-6ae3d195e07d]
2019-12-03T15:18:19.023107+00:00 app[web.1]: [aa0374eb-bab1-40cc-b40b-6ae3d195e07d] ActionView::Template::Error (Webpacker can't find application in /app/public/packs/manifest.json. Possible causes:
2019-12-03T15:18:19.023109+00:00 app[web.1]: 1. You want to set webpacker.yml value of compile to true for your environment
2019-12-03T15:18:19.023111+00:00 app[web.1]: unless you are using the `webpack -w` or the webpack-dev-server.
2019-12-03T15:18:19.023114+00:00 app[web.1]: 2. webpack has not yet re-run to reflect updates.
2019-12-03T15:18:19.023116+00:00 app[web.1]: 3. You have misconfigured Webpacker's config/webpacker.yml file.
2019-12-03T15:18:19.023118+00:00 app[web.1]: 4. Your webpack configuration is not creating a manifest.
2019-12-03T15:18:19.023120+00:00 app[web.1]: Your manifest contains:

{
 "application.js": "/packs/js/application-2a0e2c932678ebbf2ae7.js",
"application.js.map": "/packs/js/application-2a0e2c932678ebbf2ae7.js.map",
"entrypoints": {
 "application": {
"js": [
"/packs/js/application-2a0e2c932678ebbf2ae7.js"
],
 "js.map": [
 "/packs/js/application-2a0e2c932678ebbf2ae7.js.map"
 ]
 },
 "server_rendering": {
 "js": [
 "/packs/js/server_rendering-eb794d024d4852e8ab55.js"
],
 "js.map": [
 "/packs/js/server_rendering-eb794d024d4852e8ab55.js.map"
 ]
}
 },
"server_rendering.js": "/packs/js/server_rendering-eb794d024d4852e8ab55.js",
"server_rendering.js.map": "/packs/js/server_rendering-eb794d024d4852e8ab55.js.map"
}

...

<%= stylesheet_link_tag    'application', media: 'all', 'data-turbolinks-track': 'reload' %>
<%= stylesheet_pack_tag 'application', media: 'all', 'data-turbolinks-track': 'reload' %>
<%= javascript_pack_tag 'application' %>

<link href="https://fonts.googleapis.com/css?family=Open+Sans|Roboto+Slab:700&display=swap" rel="stylesheet">

2019-12-03T15:18:19.023193+00:00 app[web.1]: [aa0374eb-bab1-40cc-b40b-6ae3d195e07d] app/views/layouts/application.html.erb:10

Webpacker.yml

    # Note: You must restart bin/webpack-dev-server for changes to take effect

    default: &default
      source_path: app/javascript
      source_entry_path: packs
      public_root_path: public
      public_output_path: packs
      cache_path: tmp/cache/webpacker
      check_yarn_integrity: false
      webpack_compile_output: false

      # Additional paths webpack should lookup modules
      # ['app/assets', 'engine/foo/app/assets']
      resolved_paths: ['app/assets']

      # Reload manifest.json on all requests so we reload latest compiled packs
      cache_manifest: false

      # Extract and emit a css file
      extract_css: false

      static_assets_extensions:
        - .jpg
        - .jpeg
        - .png
        - .gif
        - .tiff
        - .ico
        - .svg
        - .eot
        - .otf
        - .ttf
        - .woff
        - .woff2

      extensions:
        - .jsx
        - .vue
        - .mjs
        - .js
        - .sass
        - .scss
        - .css
        - .module.sass
        - .module.scss
        - .module.css
        - .png
        - .svg
        - .gif
        - .jpeg
        - .jpg

    development:
      <<: *default
      compile: true

      # Verifies that correct packages and versions are installed by inspecting package.json, yarn.lock, and node_modules
      check_yarn_integrity: true

      # Reference: https://webpack.js.org/configuration/dev-server/
      dev_server:
        https: false
        host: localhost
        port: 3035
        public: localhost:3035
        hmr: false
        # Inline should be set to true if using HMR
        inline: true
        overlay: true
        compress: true
        disable_host_check: true
        use_local_ip: false
        quiet: false
        headers:
          'Access-Control-Allow-Origin': '*'
        watch_options:
          ignored: '**/node_modules/**'

    test:
      <<: *default
      compile: true

      # Compile test packs to a separate directory
      public_output_path: packs-test

    production:
      <<: *default

      # Production depends on precompilation of packs prior to booting for performance.
      compile: true

      # Extract and emit a css file
      extract_css: true

      # Cache manifest.json for performance
      cache_manifest: true

Answer 1

It looks like there's no application.css in your manifest.json which means you might not be importing any css from within your Webpack javascript files.

If that's all true, then you can fix the error in production by one of the following:

• Quick (temporary) fix: Add extract_css: false to your production block in config/webpacker.yml (which would mimic your local environments)
• If you don't want to compile css with Webpacker: Remove <%= stylesheet_pack_tag 'application' %> from your application layout
• If you want to compile some css with Webpacker: Import at least one css file from your Webpack javascript

Answer 2

If you are using Rails 6+ with webpacker then due to the latest rails 6 update the both javascript and css files are moved under app/javascript instead of app/assets.

app/javascript:
  ├── packs:
  │   # only webpack entry files here
  │   └── application.js
  └── src:
  │   └── application.css
  └── images:
      └── logo.svg

But if you have upgraded from older version to new version but still want to compile CSS from app/assets/stylesheets folder then follow the below tweaks:

1. update the below in config/webpacker.yml for webpack to include app/assets in the resolved path.

// config/webpacker.yml

resolved_paths: ['app/assets']

2. copy below line to app/javascript/packs/application.js.

// app/javascript/packs/application.js

import 'stylesheets/application'

This should fix your CSS compilation issue when you run bin/webpack-dev-server.