Angular dirrectives and ng-class

Suppose a directive with html e.g. in component.html like

<div class="text-field"
     ng-class="{'classA': varA}">
....
</div>

then I'm trying to do...

<component ng-class="{'classB': varB}"></component>

After that in ng-class I'm having something like {'classB': varB} {'classA': varA} that is obviously not working.

The problem is that I cannot change component.html but I need to add/remove class on keyup (depending on criteria). Is it possible to add ng-class in my situation or mayb there's another way to add/remove class?

Try adding another directive that adds the class via script, something like

.directive('myDir',function(){
  return {
    scope:{
     varB:'='
    },
    link: function($scope,$element){
      if($scope.varB)
        $element.addClass('classB');
    }
  }
});

and use like:

<component ng-class="{'classB': varB}" my-dir var-b="something"></component>