Why does my Backbone model hasChanged() always return false?

Question

Given the following snippet:

var m = new Backbone.Model({
    name: 'joshua'
});

m.set('name', 'something else');

If I now call m.hasChanged() or m.hasChanged('name') I get false . Why? My understanding is that both of these should return true .

m.changedAttributes() also returns false.

Here is a fiddle that illustrates what I'm doing, and expecting: http://jsfiddle.net/9cvVv/88/

EDIT: It seems that unless you pass { silent: true; } { silent: true; } to the set() method then it will fire the change event on your model which clears out the changedAttributes() , etc. Essentially these properties only track changes since the last time the change event was triggered.

I read that in the documentation but didn't really understand it at first.

This doesn't seem very useful to me. I would appreciate any explanation of why this works the way it does and the best way to achieve the result I want. (Passing around {silent: true; } and giving up usage of the change event seems like a waste.)

Answer 1

Unless you pass { silent: true; } { silent: true; } to the set() method then it will fire the change event on your model which clears out the changedAttributes() , etc. Essentially these properties only track changes since the last time the change event was triggered.

So the answer is to call this instead:

m.set('name', 'something else', {silent: true})

Answer 2

This post is premised on the previous behavior of older versions of Backbone. hasChanged does now ( as of 0.9.10 ) always returns true after set(..) is called. The silent flag no longer has any effect.

This is confusingly masked in the jsfiddle linked in the question which uses a CDN-hosted copy of backbone.js which always uses the latest version. Here's some updated jsfiddles showing the change in behavior:

• jsfiddle using Backbone 0.9.2
• jsfiddle using Backbone 0.9.10

Answer 3

Model.set() takes an attributes hash as the first argument. Try m.set({'name': 'something else'}); . Doing m.set('name', 'something') doesn't set 'name', so it never triggers the change event and your call to hasChanged() returns false . You can always inspect the current value of the attributes hash by logging out m.attributes - though accessing and manipulating it directly isn't recommended as none of the change events will fire that way.