Cloud Firestore: Update fields in nested objects with dynamic key

I found the solution inspired by a firebase solution (replacing "/" by ".").

var usersUpdate = {};
usersUpdate[`favorites.${key}.color`] = true;

db.collection("users").doc("frank").update(usersUpdate);

This solution worked for me:

db.collection('users').doc('frank').update({
  [`favorites.${key}.color`]: true
});

Just a note about a potential pitfall: After discovering that you can update nested fields using a dot syntax, I tried using set() the same way, since I needed it to work whether the object already exists or not:

var updateObj = {['some.nested.property']: 9000};
docRef.set(updateOb, {merge: true});

Unfortunately, this doesn't work – it sets a property whose key is some.nested.property instead. Inconsistent, but okay.

Fortunately, it appears that set(updateObj, {merge: true}) does a deep merge, so if you construct your update object as a fully nested object, your nested objects will also be properly merged:

// create the object
db.doc('testCollection/doc').set({p1: {['p2']: {p3: true}}}, {merge: true})
// update the existing object
db.doc('testCollection/doc').set({p1: {['p2']: {p4: true}}}, {merge: true})

// result is now this:
{ p1: { p2: { p4: true, p3: true } } }