Clarify difference between "extends" and "superclass" properties of types

[felixlinker]

In the documentation, there is only mentioned how to use the superClass property of a type. But the code and further examples relating to bpmn-js also show there is a extends property.

It would be great to know, how those differ as their naming is rather ambiguous.

[nikku]

extends is basically a mechanism to plug-in additional meta-data to known elements without modifying the existing inheritance hierarchy.

Think about it as a plug in mechanism from the side.

Instead of doing this:

A
^
|
B

You may provide additional type information to A by extending it with a type definition C:

A <-- C <extends>

This means A is now of type C, too and knows all properties of C.

As part of model checks you may now ask whether your dealing with C if you got an instance of A at hand: isA(a, C) === true.

Understandable?

[felixlinker]

Understandable! But what exactly is the difference between extends and superClass now? As far as I understand, in both cases (B extends A/B has-superClass A) isA(b, A) would result into true wouldn't it?

[nikku]

That is correct. The difference is that extending is implicit while inheriting via superClass is explicit.

Take the example of constructing an instance of A: In the extends case constructing A will give you all the properties defined in C, too. In the superClass case, you'd need to instantiate B to get all the properties defined by it.

[nikku]

Feel free to ask more questions, if that helps you understand. I'll leave this issue open until we add the actual documentation for extends.

[felixlinker]

Ok, cool. Along with the new documentation I think I got it! Thanks for the quick response.