Page 15 shows these imports for the conversion of application.js to application.ts
import * as ActiveStorage from "@rails/activestorage"
import Rails from "@rails/ujs"
import * as Channels from "channels"
import Turbolinks from "turbolinks"
However I had to change the Channels import to the following to get a build:
// from @rails/actioncable not channels
import * as Channels from "@rails/actioncable"
Environment: Ruby 2.7.1 Rails 6.0.3.2 Ubuntu 20 LTS