How to add Bootstrap to a Rails 6 project
Create a new Rails 6 project.
$ rails new awesome-project
Add bootstrap, popper.js and jquery using yarn.
$ yarn add bootstrap popper.js jquery
Copy the snippet below into config/webpack/environment.js
const webpack = require('webpack')
environment.plugins.append('Provide', new webpack.ProvidePlugin({
  $: 'jquery',
  jQuery: 'jquery',
  Rails: '@rails/ujs'
}))
Rename the application.css in app/assets/stylesheets into application.scss and copy & paste the snippet below.
@import 'bootstrap/scss/bootstrap';