Delete Field in Firestore Document

You can try as shown below:

// get the reference to the doc
let docRef=this.db.doc(`ProfileUser/${userId}/followersCount/FollowersCount`);

// remove the {currentUserId} field from the document
let removeCurrentUserId = docRef.update({
    [currentUserId]: firebase.firestore.FieldValue.delete()
});

This worked for me. (also worked to delete field with null value)

document.ref.update({
  FieldToDelete: admin.firestore.FieldValue.delete()
})

For some reason the selected answer (firebase.firestore.FieldValue.delete()) did not work for me. but this did:

Simply set that field to null and it will be deleted!

// get the reference to the doc
let docRef=this.db.doc(`ProfileUser/${userId}/followersCount/FollowersCount`);

// remove the {currentUserId} field from the document
let removeCurrentUserId = docRef.update({
    [currentUserId]: null
});