How to check if a method argument of a directive is specified in AngularJS?
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.
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); } }