Import javascript class in the standard asset pipeline

application.js

// app/javascript/packs/application.js

import Foobar from '../foobar/foobar'

window.Foobar = Foobar

application.js.coffee

# app/assets/javascripts/application.js.coffee

#= require jquery
#= require jquery_ujs
#= require_tree .

browser_console.js

// Test this in your browser
var test = new Test()
test.hello()

var foobar = new Foobar()
foobar.hello()

foobar.js

// app/javascript/foobar/foobar.js

export default class Foobar {
  hello() {
    console.log("Hello from Foobar")
  }
}

index.html.slim

// app/views/home/index.html.slim 

= javascript_pack_tag "application"

javascript:
  var foobar = new Foobar();
  foobar.hello();

test.js.coffee

# app/assets/javascripts/test.js.coffee

class @Test # For coffeescript @Test is the same as window.Test
  constructor: () ->
    console.log("I'm Test")

    @foobar = new window.Foobar()

  hello: ->
    @foobar.hello()