I wrote this guide few months ago, check it out. ~ https://medium.com/wecode-ng/how-to-install-bootstrap-on-phoenix-1-4-project-c8aa724dcdeb

Ah right, you can just add import 'bootstrap'; in app.js

What I’ve done so far is to use package.json. In the case of jQuery I’ve added it on assets/package.json in this way

"dependencies": {
    "phoenix": "file:../deps/phoenix",
    "phoenix_html": "file:../deps/phoenix_html",	
    
    "jquery": "^3.4.1"
}

and then on the terminal

assets$ npm install

The jQuery library is then downloaded in assets/node_modules and available to be imported on any javascript file. If I create a script like assets/js/custom.js, I can then import jQuery like so

// assets/js/custom.js
import jQuery from "jquery"

jQuery(document).ready(function($){
  ...
})

and to include the script into the assets pipeline I have to import it in app.js

// assets/js/app.js
import "./custom"