Webpacker by default uses yarn as a package manager for node_modules
To add any new JS module you can use yarn:
yarn add bootstrap material-uiTo add a new JS module that will only be available to local development:
yarn add --dev browser-syncBe 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.