How to assign json value to variable in Vue.js?

I use Firebase to store the database, and have a object currentWeek with a value off the current week.

I want to store the value of this object in a variable to call it.

Currently I'm using this to get the current week, but I want to get the current week from the database:

    <template>
  <div id="app">
      <div v-if="ligler[1]">
      <h3>{{ligler[1].weeks[currentWeek].day}}</h3>
  </div>
</template>

<script>
import Firebase from 'firebase'

let config = {
    apiKey: "xxxx",
    authDomain: "xxx.firebaseapp.com",
    databaseURL: "https://xxx.firebaseio.com"
}

let app = Firebase.initializeApp(config);
let db = app.database();

let ligRef = db.ref('ligler');

export default {
  name: 'app',
  firebase: {
    ligler: ligRef
  },
  data() {
      return {
        currentWeek: 18
      }
  }
}
</script>

I tried this, but it doesn't work:

data() {
      return {
        currentWeek: ligler[1].currentWeek
      }
  }

The database look like:

How can I assign the value of currentWeek to a variable in vue.js? To use it in the code as [currentWeek]:

<h3>{{ligler[1].weeks[currentWeek].day}}</h3>