How to override individual properties of an Object-based attribute in YUI3 Base subclasses

definition.js

var MyClass = Y.Base.create('my-class', Y.Base, [], {
  _mergeProps: function (value, prop) {
      var cur = this.get(prop);
      if (Y.Lang.isUndefined(cur)) {
        cur = this.constructor.ATTRS[prop].value;
      }
      return Y.Lang.isObject(value) ? Y.merge(cur, value) : Y.Attribute.INVALID_VALUE;
  }
}, {
  ATTRS: {
    dict: {
      value: {
        foo: 'bar',
        test: false
      },
      setter: '_mergeProps'
    }
  }
});

implementation.js

var instance = new MyClass({
  dict: {
     test: true
  }
});
instance.get('dict'); // { foo: 'bar', test: true }
instance.set('dict', { thirdVal: 'yes' });
instance.get('dict'); // { foo: 'bar', test: true, thirdVal: 'yes' }
delete instance.get('dict').thirdVal;
instance.get('dict'); // { foo: 'bar', test: true }

You might want to use this.reset( prop ) instead of setting things back to the static default values: https://gist.github.com/867093

Since the reset method sits within a validator, this.reset(prop) throws a too much recursion error.

Checking for undefined is important because when the class is initialized, the current value of the attr is undefined - hence why I go back to the static defaults. It does make sense to check that the new value is an object though, as setting the property to, e.g. a String will cause unexpected results.