As documented at the bootstrap website, the actions that require javascript are not auto-included and you need to call them yourself. I.E., you could add something like this to your app.js for your specific example:
import $ from "jquery"
// Some bootstrap things need this, not most thankfully, keep disabled unless required, this one might require it?
// global.jQuery = require("jquery")
// Make sure you included bootstrap js in your npm and brunch config files or this will not work
global.bootstrap = require("bootstrap")
// Do the stuff to the existing html at page load time:
$(document).ready(() => {
// This is optional if you want to do it on 'something other than default'
$('[data-toggle="collapse"]').collapse()
}
Or something like that (I don’t use collapse, just dropdown and tooltip, and I’m fazing those out for other solutions over time).
I.E. Need to include bootstrap’s javascript. 
