app/javascript/controllers/fontawesome.js
import { dom, library } from '@fortawesome/fontawesome-svg-core'
import { faCalendar } from '@fortawesome/free-solid-svg-icons'
import { faCheckSquare, faSquare } from '@fortawesome/free-regular-svg-icons'
library.add(
faCalendar,
faCheckSquare, faSquare
)
dom.watch()