directives with isolated scope and a optional function '&?' should be returned as undefined if the function is missing · Issue #6404 · angular/ angular.js · GitHub
AngularJS custom directive and isolated scope; how to pass data to and from directive scope | by Han Lee | Medium
chapter: Chapter-6-The-Directive-In-Detail / AngularJS Book
AngularJs Directive Isolated Scopes
AngularJS - Isolate Scope "@" - Thinkster
Infographic: Understanding AngularJS Isolated Scope - One Hungry Mind
Understanding Scopes in AngularJS Custom Directives | Infragistics Blog