Today I was installing Webpacker in an existing app that already has a package.json. (The reasons for the existing package.json are boring and probably irrelevant.) Turns out if there’s conflicts between webpacker’s needs and the existing package.json, then it’s possible for the yarn add @rails/webpacker@5.1.1 and yarn add --dev webpack-dev-server commands run during the install process to fail. The webpack installer does not respond to this failure, just proceeds.
After correcting my package.json to be webpacker-compatible, I re-ran yarn install. Naive belief: this would be sufficient to install everything webpacker needed. Actual result: webpack and webpack-dev-server were not installed, leading to error Command "webpack-dev-server" not found. when running bin/webpack-dev-server.