Cannot read property 'prototype' of undefined; CoffeeScript inheritance with Require.js

Question

I'm using Require.js and CoffeeScript. I have what I find to be a rather strange error.

I have the following definition:

define ->
    {
        Node: class

        ChildNode: class extends @Node

        Term: class

        ChildTerm: class extends @Term

    }

This raises the following error:

Uncaught TypeError: Cannot read property 'prototype' of undefined

However, the following both work fine:

define ->
    {
        Node: class

        ChildNode: class extends @Node

    }



define ->
    {
        Node: class

        ChildNode: class extends @Node

        Term: class

        ChildTerm: class extends @Node

    }

What is wrong with my code such that I can't extend Term? I can't see anything different between it and Node.

score 3 · Accepted Answer · edited May 23 '17 at 10:26

However, the following both work fine

Actually they don't. Have a look at what code they're producing:

// [simplified, obviously]
function __extends(child, parent) {
  child.prototype = Object.create(parent.prototype);
  return child; //                ^^^^^^^^^^^^^^^^
}
define(function() {
  return {
    Node: function _Class() {},
    ChildNode: __extends(function _Class() {}, this.Node)
  } //                                         ^^^^^^^^^
})

You cannot have Self-references in object literal declarations.

So why does subclassing Term like this throw an error while with Node it does not?

Because the this does refer to the global object. And that does actually have a Node property already: the DOM Node interface constructor (which you cannot subclass, though). With Term however, you are passing undefined to __extend().

Cannot read property 'prototype' of undefined; CoffeeScript inheritance with Require.js

1 Answers1