title: What does '=?' mean in the scope of a directive in AngularJS?
body: <p>I have the following code and in that, codeValue property is set to '=?'. Please note the question mark after the equal to symbol. </p>
  
  <p>What does '=?' do?</p>
  
  <pre><code>app.directive('questionMark', function () {  
     return {  
        restrict: 'E',
        scope: {  
           codeValue: "=?"  
        },  
        template: 'Hello world'  
     };  
  }); 
  </code></pre>