I'm learning AngularJS and I see the tag "li" is associated with the ngClass directive which has as "{marked: nav.estoy ( '/')}" values. **My question is why the value inside the brackets has the form x: y.** I am that "nav.estoy ('/') is defined in the controller method but do not understand why the response of this method is to":"

```
<body ng-app="app">
<nav ng-controller="navCtrl as nav">
    <ul>
        <li ng-class="{marcado: nav.estoy('/')}">
            <a href="#/">Datos personales</a>
        </li>
    </ul>
</nav>

```

...