Yarn

Webpacker by default uses yarn as a package manager for node_modules

Add a new npm module

To add any new JS module you can use yarn:

yarn add bootstrap material-ui

Add an npm module to devDependencies

To add a new JS module that will only be available to local development:

yarn add --dev browser-sync

Be careful not to add any build or app related JS modules in this fashion. Adding JS modules to devDependencies will block them from being installed in any production environment.

Docs from JS modules may instruct you to use --dev or devDependencies, but this is generally under the assumption that you are using a node.js workflow.