yarn add bootstrap jquery popper.js postcss-loader sass-loader
app/javascript/packs/application.js
import 'bootstrap'
import '../stylesheets/application'
app/javascript/packs/stylesheets/application.scss
@import "custom";
@import "~bootstrap/scss/bootstrap";
app/views/layouts/application.html.haml
= javascript_pack_tag 'application'
= stylesheet_pack_tag 'application'