Steps to reproduce Using rails 5.1.0-beta1, run yarn add actioncable@5.1.0-beta to get actioncable in your application to be 5.1.0-beta1. Create a file in frontend/javascripts that will be loaded by webpacker that imports actioncable as such import * as ActionCable from 'actioncable' (this is typescript syntax) Expected behavior ActionCable should be loaded normally and no problem should occur as