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.

<!DOCTYPE html>
<html ng-app="app">
<head>
    <title>Simple directive</title>

    <script src="js/lib/angular/angular.js"></script>

    <script type="text/javascript">
        var app = angular.module('app', []);

        app.controller('TestController', function($scope) {

            $scope.doSomething = function(param) {
                alert('Something called with: ' + param);
            }
        })

        app.directive('myDirective', function() {
            var ret = {
                restrict: 'E',
                scope: {
                    user: '@',
                    callback: '&'       // bound a function from the scope
                },
                template: '<div>Hello {{user}}<button ng-show="hasCallback()" ng-click="callback({userData: user})">Callback</button>',
                controller: function($scope) {
                    $scope.hasCallback2 = function() {
                        var t = typeof $scope.callback;
                        return t == 'function';
                    }

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

    </script>
</head>

<body ng-controller="TestController">

<my-directive user="cat" callback="doSomething(userData)"></my-directive>
<my-directive user="dog" callback="doSomething(userData)"></my-directive>
<my-directive user="pig"></my-directive>

</body>

</html>

My question is:

How can I control visibility of button inside template? I'd like to hide it if callback attribute not specified in custom tag (see 3rd my-directive tag). When I check typeof of callback, I always get 'function' and angular.isDefined(...) also returns true.

Check the attrs array in the directive's link function

Gruff Bunny
– Gruff Bunny

2014-02-21 13:01:20 +00:00
Commented Feb 21, 2014 at 13:01 — Gruff Bunny
– Gruff Bunny, Commented Feb 21, 2014 at 13:01

Martin · Accepted Answer · 2016-01-05 12:16:51Z

91

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.

edited Jan 5, 2016 at 12:16

Martin

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answered Nov 26, 2015 at 10:17

Bulki S Maslom

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4 Comments

Rebecca Over a year ago

This worked perfectly and is a much more elegant solution. Should be the correct answer.

fracz Over a year ago

There is not a single word about it in the docs.

noherczeg Over a year ago

This does not work in 1.3.18, it always wires in a wrapper function no matter if it is defined in the DOM or not.

vip Over a year ago

Docs: docs.angularjs.org/api/ng/service/$compile Directive Definition Object -> scope section

Khanh TO · Accepted Answer · 2014-02-21 13:17:03Z

44

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

edited Feb 21, 2014 at 13:17

answered Feb 21, 2014 at 13:04

Khanh TO

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2 Comments

Stalinko Over a year ago

helped me, but instead of angular.isDefined I prefer vanilla-js attrs.hasOwnProperty('callback');

Khanh TO Over a year ago

@Stalinko: if you want to use vanila-js for some reason, a simple check for undefined should work: if (typeof attrs.callback === "undefined")

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