Question

What’s the difference between ember.js Object methods extend and create?

TLDR: You will find the answer in Ember guides: classes and instances.

Sometimes I see that one in the examples and sometimes the other. In particular, what’s the difference between Em.Application.extend({}) and Em.Application.create({})?

If I declare my app like this, what does it mean?

Ember.Application.create({
    MyController : Ember.ArrayController.extend({

    }),
});

How can I access the instance of MyController? Do I need to create it somehow? I need to push some data into it.

Answer 1

The dead simple answer is that extend defines a new JS Class which inherits from the class you’re extending, but does not create an instance of that class. create creates an instance of the class.

Em.Application is a particular case, because you’re creating a namespace, not an object instance. I don’t know when you’d ever want to extend Em.Application.

App = Em.Application.create(); // I have a new Em.Application namespace
App.A = Em.Object.extend(); // I have defined a new class, App.A, 
                            // which inherits from Em.Object
var a = App.A.create();  // a now contains an instance of App.A.

I’d suggest you read “Naming Conventions“, too.

ETA: And “Understanding Ember Objects“, as suggested in zaplitny’s comment.