changedAttributes() behaving weird

[jeffijoe]

I am using Model.changedAttributes() to write a computed field helper, but there's an issue where changedAttributes returns false when not expected (at least not by me).

Reproduced in this pen.

console.clear();
let Backbone = window.Backbone;
var m = new Backbone.Model();

let log = (txt, obj) => document.write(txt + ' ', JSON.stringify(obj)+ '<br/>');

let lastChange;
m.on('change', () => {
  log('Before', lastChange);
  lastChange = m.changedAttributes(lastChange);
  log('After', lastChange);
});

m.set('search', 'some text');
// Why does this emit "false"? It was changed!
m.set('search', '');
m.set('search', 'some other text');

I need the lastChange because there are multiple instances of that code on a model instance. I use it to determine dependency changes (for computed fields).

Is this expected behavior?

[jridgewell]

Looking into it, when passing the diff object during a change or change:* event, we compare against the previous attributes and not the current. Meaning, you're comparing {search: 'some text'} against {search: 'some text'}.

I think what you want is closer to model.changedAttributes(), or _.pick(model.changedAttributes(), 'search');.

---

I'm not sure that the previous attributes behavior is what's expected here. Someone else want to weigh in?

[skeptic35]

Stumbled upon this very problem today and imho it is a bug. According to documentation the change event is triggered "when a model's attributes have changed". "have changed" - not "are about to change". So a call to changedAttributes in a change handler should imho compare to the current attributes, not the previous ones. Alas the internal _changing property is reset to false only after the change event is triggered. Thus changedAttributes, when being called with the diff parameter, compares to the previous attributes.