How to check if a method argument of a directive is specified in AngularJS?

Question

I\'ve created a custom directive which contains a button. This button calls a method from parent scope specified by \'callback\' attribute.

Answer 1

Using '&?' returns undefined if the attribute has not been set.

'&' = callback function is defined always.

'&?' = callback function is defined only when attribute is defined in html template.

bindToController: {
    callback: '&?'
},
controller: function() {
    if (this.callback === undefined) {
        // attribute "callback" was not defined
    }
}

Note: Works in Angular 1.4.8. I'm not sure if it works in older versions.

Answer 2

Looking at angularjs source code, I see this:

case '&':
    parentGet = $parse(attrs[attrName]);
    isolateScope[scopeName] = function(locals) {
         return parentGet(scope, locals);
    };
    break;

The parentGet is the bound function expression. Unfortunately, this is a local variable which is only available to the function assigned to isolateScope[scopeName] via closure.

Instead of trying to find a way to get that variable, a simple solution is just to check the attrs. Try:

link: function(scope,elem,attrs) {

      scope.hasCallback = function() {
        return angular.isDefined(attrs.callback);
      }
    }

DEMO