Bootstrap 4 in Rails 5.2 with Yarn

Install libraries with yarn

$ yarn add jquery
$ yarn add popper.js
$ yarn add bootstrap

These get installed in the node_modules directory. Here are the relevant paths:

node_modules
├─ bootstrap
│  ├─ dist
│  │  └─ js
│  └─ scss
├─ jquery
└─ popper.js
   └─ dist
      └─ umd

Import styles in application.scss

@import "bootstrap/scss/bootstrap";

I like to import the source SASS files directly so I have access to all the variables.

Require JavaScript in application.js

//= require jquery
//= require popper.js/dist/umd/popper
//= require bootstrap/dist/js/bootstrap