" =? " in Angular Directive Isolate Scope

Question

scope: {
    someProperty: "=?"
    somePropertyTwo: =?Wheeeeee
}

What does "=?" do? I can't find the answer anywhere. I understand that using scope: {} (or scope: true) gives the directive a new scope, with the former being an isolate scope and the latter being one-way binded to parent Ctrl (Ctrl of the page/view on which the directive is used). I understand that:

someProp: @X //will one-way bind someProp to parent Ctrl's X
someProp: =X //two-way
someProp: &X() //some space magic for binding methods

However, I don't understand how/why " =? " is used.

Relevant articles (that only cover the first three): http://www.undefinednull.com/2014/02/11/mastering-the-scope-of-a-directive-in-angularjs/ http://weblogs.asp.net/dwahlin/creating-custom-angularjs-directives-part-2-isolate-scope

edit: Is it the same as these?

? - Attempt to locate the required controller or pass null to the link fn if not found. ^ - Locate the required controller by searching the element and its parents. Throw an error if not found. https://docs.angularjs.org/api/ng/service/$compile#-require-

I.e. null is passed if whatever property isn't found?

Answer 1

It just means a two-way binding is optional. If you define a property by using = then you must provide it with a valid binding. From $compile documentation:

(...) If the parent scope property doesn't exist, it will throw a NON_ASSIGNABLE_MODEL_EXPRESSION exception. You can avoid this behavior using =? or =?attr in order to flag the property as optional. If you want to shallow watch for changes (i.e. $watchCollection instead of $watch) you can use =* or =attr (=? or =*?attr if the property is optional).

Answer 2

"=?" is just optional "=".

They are identical except that if you miss this property when you use this directive, no error will occur, the scope will be used as normal internally.