How to set a default value in an AngularJS Directive Scope?

I think a better way to check that value is look for an undefined value like this:

controller: function($scope){
    if (angular.isUndefined($scope.allowSomething))
        $scope.allowSomething = true;
}

I had the same issue once and this worked for me. I think the best method is to use angular's methods for handling things.

This is how I have been doing it:

html:

<my-directive allow-something="false"></my-directive>
<my-directive></my-directive>

directive:

angular
.module('app')
.directive('myDirective', function() {

var defaults = {
  allowSomething: true
};

return {
  restrict: 'E',
  templateUrl: '',
  scope: {},
  compile: function() {
    return {
      pre: function(scope, el, attrs) {
        scope.allowSomething = attrs.allowSomething || defaults.allowSomething;
      },

      post: function(scope, el) {
        // this is link
        // scope.allowSomething = default or whatever you enter as the attribute "false"
      }
    };
  }
};
}

The pre is fired before anything happens then the post is like the link function. This has allowed me to do dynamic things based on the attributes I set.