errors creating a custom web component with coffeescript

Question

I am trying to create a custom HTML5 web component in a Rails application, but I am getting errors instantiating the Javascript object.

Version info:
Rails 4.2.11.3
ruby 2.5.8
coffee-rails 4.1.1

I've reproduced the issue in a minimal Rails app, available at https://github.com/fredwillmore/coffee_test

Here's the coffeescript file that creates the component:

class ThingDoer extends HTMLElement
  # constructor: ->
  #   super()

customElements.define("thing-doer", ThingDoer);

This is what the coffeescript transpiles to:

ThingDoer = (function(superClass) {
    extend(ThingDoer, superClass);

    function ThingDoer() {
      return ThingDoer.__super__.constructor.apply(this, arguments);
    }

    return ThingDoer;

  })(HTMLElement);

customElements.define("thing-doer", ThingDoer);

Errors:

with no constructor defined, I get the following error:

Uncaught TypeError: Failed to construct 'HTMLElement': Please use the 'new' operator, this DOM object constructor cannot be called as a function.

same with a constructor that just calls "super":

Uncaught TypeError: Failed to construct 'HTMLElement': Please use the 'new' operator, this DOM object constructor cannot be called as a function.

I tried a constructor that just calls new HTMLElement :

Uncaught TypeError: Illegal constructor

What am I missing?

Answer 1

The compiled script isn't suitable as the class has been compiled to an ES5-style class which is not compatible with Custom Elements API.

Instead, you can write a little bit of raw JavaScript to generate a real class, then augment more methods to it using the CoffeeScript's Prototypal Inheritance syntax :

ThingDoer = `class ThingDoer extends HTMLElement { constructor() { super(); this.initialize(); } }`

ThingDoer::initialize = ->
  @innerHTML = 'meow'

customElements.define "thing-doer", ThingDoer