Get Data from State React-Native Javascript

This is puzzling me why I can't get data from the state, I have tried many things but not had no luck.

I'm getting data from a firebase database and I am trying to store the data into a state, then I can access that data from the state.

Any suggestions will be appreciated.

Thank you

state = {
   isDateTimePickerVisible: false,
   mode: "time",
   minuteInterval: 30,
   selectedDate: {},
   isLoading: true,
   syllabus: [{value: 'Banana'}],
   users: [{
     value: 'Banana',
     app_id: 'apple',
   }],
  timeSlots:[{
     name: 'test',
     time: '123',
  }],
};

testStore() {

let returnArr = [];

  db.ref('bookings/' + this.user).child("22-09-2018").once('value').then(function(snapshot) {
     let data = snapshot.val();
     let timeSlot = Object.values(data);
     console.log("testStore: " + timeSlot[0].name);
     //gets data from the firebase database -- testStore: james
     dt = timeSlot;

     this.setState({
       timeSlots: dt
     });

}.bind(this));
  this.testState();
}

testState() {
  console.log("test state " + this.state.timeSlots[0].name);
  //Get A red screen with error undefined is not an object(evaluating 'this.state.timeSlots['name']) 
}

    onDayPress(day) {

        console.log("day press " + day.dateString);
        this.testStore();
        var st = day.dateString;
        var dt = new Date(st);

        var today = dt;
        var dd = today.getDate();
        var mm = today.getMonth() + 1; 
                                                

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