title: " =? " in Angular Directive Isolate Scope
body: <pre><code>scope: {
      someProperty: "=?"
      somePropertyTwo: =?Wheeeeee
  }
  </code></pre>
  
  <p>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:</p>
  
  <pre><code>someProp: @X //will one-way bind someProp to parent Ctrl's X
  someProp: =X //two-way
  someProp: &amp;X() //some space magic for binding methods
  </code></pre>
  
  <p>However, I don't understand how/why " =? " is used.</p>
  
  <p>Relevant articles (that only cover the first three):
  <a href="http://www.undefinednull.com/2014/02/11/mastering-the-scope-of-a-directive-in-angularjs/" rel="noreferrer">http://www.undefinednull.com/2014/02/11/mastering-the-scope-of-a-directive-in-angularjs/</a>
  <a href="http://weblogs.asp.net/dwahlin/creating-custom-angularjs-directives-part-2-isolate-scope" rel="noreferrer">http://weblogs.asp.net/dwahlin/creating-custom-angularjs-directives-part-2-isolate-scope</a></p>
  
  <p>edit: Is it the same as these?</p>
  
  <p>? - 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.
  <a href="https://docs.angularjs.org/api/ng/service/" rel="noreferrer">https://docs.angularjs.org/api/ng/service/</a>$compile#-require-</p>
  
  <p>I.e. null is passed if whatever property isn't found? </p>