The problem

tl;dr to the solution that worked for me

I was trying to get Ruby on Jets up and running and ran into webpacker errors including “CLI for webpack must be installed” (and others) all the way to “Invalid configuration object”.

On creating a new app with jets (3.0.8) with npm 7.11.2, node 16.1.0, and yarn 1.22.5 and creating a basic blog CRUD app like follows:

 jets new blog_app --database=postgresql
 cd blog_app
 jets generate scaffold post title:string post:text
 jets db:create
 jets db:migrate
 jets server

I get an error with browsing to localhost:8888/posts:

ActionView::Template::Error at /posts

Webpacker can't find application.js in /mnt/c/Users/twill/projects/jets/blog_app/public/packs/manifest.json. Possible causes: 1. You want to set webpacker.yml value of compile to true for your environment unless you are using the `webpack -w` or the webpack-dev-server. 2. webpack has not yet re-run to reflect updates. 3. You have misconfigured Webpacker's config/webpacker.yml file. 4. Your webpack configuration is not creating a manifest. Your manifest contains: { }

On the back end, I could see:

[Webpacker] Compilation failed:
warning package.json: No license field
CLI for webpack must be installed.
webpack-cli (https://github.com/webpack/webpack-cli)

We will use "npm" to install the CLI via "npm install -D webpack-cli".
Do you want to install 'webpack-cli' (yes/no):

Adding dependencies one by one

I added webpack-cli with yarn add webpack-cli and:

[webpack-cli] Failed to load '/mnt/c/Users/twill/projects/jets/blog_app/config/webpack/development.js' config
[webpack-cli] Error: Cannot find module '@rails/webpacker'

Ok, add yarn add @rails/webpacker and run jets server and reload the page:

[Webpacker] Compilation failed:
 warning package.json: No license field
 [webpack-cli] Invalid configuration object. Webpack has been initialized using a configuration object that does not match the API schema.
 configuration.node should be one of these: false | object { __dirname?, __filename?, global? } -> Include polyfills or mocks for various node stuff. Details: configuration.node has an unknown property 'dgram'. These properties are valid:
 object { __dirname?, __filename?, global? }
 -> Options object for node compatibility features.
 configuration.node has an unknown property 'fs'. These properties are valid:
 object { __dirname?, __filename?, global? }
 -> Options object for node compatibility features.
 configuration.node has an unknown property 'net'. These properties are valid:
 object { __dirname?, __filename?, global? }
 -> Options object for node compatibility features.
 configuration.node has an unknown property 'tls'. These properties are valid:
 object { __dirname?, __filename?, global? }
 -> Options object for node compatibility features.
 configuration.node has an unknown property 'child_process'. These properties are valid:
 object { __dirname?, __filename?, global? }
 -> Options object for node compatibility features.  

That led me eventually to Webpack 5: configuration.node has an unknown property ‘dram’. There properties are valid: issue comment on GitHub, with the mention of rails webpacker and that “Rails’ webpacker 5.x.x is only compatible with webpack 4.x.x”

The Solution

After lots of teardowns and tweaks to the project to troubleshoot, I finally ended up with a sequence that produces a scaffold for posts that renders with webpack:

jets new blog_app --database=postgresql
cd blog_app
jets generate scaffold post title:string post:text

# jets db:drop # used while experimenting with yard packages below
jets db:create
jets db:migrate

# this is the key line that makes the currently installed @rails/webpacker work
yarn add webpack@4 
yarn add webpack-cli
yarn add @rails/webpacker
jets server