pass variable as key to firestore update code example

Example: firestore update map

let initialData = {
  name: 'Frank',
  age: 12,
  favorites: {
    food: 'Pizza',
    color: 'Blue',
    subject: 'recess'
  }
};

let updateNested = db.collection('users').doc('Frank').update({
  age: 13,
  'favorites.color': 'Red'
});