directives that have '=' binding, changing scope value causes error if not specified(optional)

[johnnyelwailer]

If I have a directive with a scope definition, and setting a value in the scope:

directive('some', function () {
  return  {
    scope: {optionalValue: '='},
    bind: function(scope) {
      scope.optionalValue = something;
    }
  }
}

And create it like

<some />

With optionalValue not specified, it yields the NON_ASSIGNABLE_MODEL_EXPRESSION error.
But I'd expect it to work in this case.

This could be solved in the "scope mode parser" by simply not applying the binding:

case '=': {
  if (attrs[attrName] == null) {
    return;
  }
   ...

Or is this intentionally? If so, maybe add a new symbol like '~' for optional attributes?

(just realized that I opened essentially the same issue 3 months ago: #1131)

[NickHeiner]

I would like to see any easy way for optional arguments / argument defaults on directive attributes as well.

[pkozlowski-opensource]

Created a jsFiddle to expose the same issue with the latest version of AngularJS: http://jsfiddle.net/yA2rn/
The use case seems to be a valid one...

[presstube]

Just chiming in that this is something that would be very useful to me as well. thanks

[bendalton]

ditto.

[fx]

+1

[ryanzec]

I also think this would be very useful. Right now I am doing this in my directives:

var optionalParameters = scope.$eval(attributes.optionalParameters);

which works ok if I don't need bi-directional binding with the parent scope.

[tschieggm]

+1

[just-boris]

That's good. Now, I make optional binding like this:

link: function(scope, element, attrs) {
    if (attrs.selected) {
        getSelected = $parse(attrs.selected);
        setSelected = getSelected.assign;
        scope.$watch(
            function watchSelected() {
                 return getSelected(scope.$parent); 
            },
            function updateSelected(value) {
                if(setSelected) {
                     setSelected(scope.$parent, value);
                }
                scope.selected = value;
            }
        );
        scope.selected = getSelected ? getSelected(scope.$parent) : false;
    }
}

This code can be replaced by simple config

scope: {
    selected: "=?"
}

[roypeled]

Please merge lrlopez's fix with the master build!

[Mischi]

+1