Question

I’m getting the error:

Uncaught TypeError: Cannot read property 'constructor' of undefined 

When declaring the following class:

 class ViewHelpers extends Backbone.Events

I can use the same syntax to extend Backbone.Router, Views, Model etc. Here is the compiled javascript which I wrote in a quick log to make sure Backbone.Events was there

__t('views').ViewHelpers = (function(_super) {

 #how i know it is definied here
 console.log(_super.trigger)

 __extends(ViewHelpers, _super);

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

 return ViewHelpers;

})(Backbone.Events);

So the line causing the error is

ViewHelpers.__super__.constructor.apply(this, arguments);

What is different about __extends() method that it would work for Backbone.View and not Backbone.Events?

Answer 1

That’s because Backbone.Events is not a “class”, so it cannot be extended, it’s a “module” that can be mixed-in into other objects (see docs here). In JavaScript terms that means that it’s not a Function, that can be called as a constructor (i.e. new Backbone.Events will throw an error), it’s just a plain JS object whose properties (methods) can be assigned to other objects to make them event dispatchers.

In CoffeeScript, you can mix-in the Backbone.Events into your objects when they are created:

class ViewHelpers
  constructor: ->
    _.extend @, Backbone.Events

Or you can just extend the class’ prototype and avoid having those methods as (own) properties of all ViewHelpers instances:

class ViewHelpers
  _.extend @prototype, Backbone.Events

These two approaches should work and let you instantiate and use ViewHelpers as event dispatchers:

vh = new ViewHelpers
vh.on 'foo', -> alert 'bar'
vh.trigger 'foo'​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​