"The operator '[]' isn't defined" error when using .data[] in flutter firestore

Change this:

name: doc.data['name'] ?? '' 

Into this:

name: doc.data()['name'] ?? '' 

data() is a method now therefore you have to add (), from the source code:

  Map<String, dynamic> data() {
    return _CodecUtility.replaceDelegatesWithValueInMap(
        _delegate.data(), _firestore);
  }

https://github.com/FirebaseExtended/flutterfire/blob/master/packages/cloud_firestore/cloud_firestore/lib/src/document_snapshot.dart#L38

For me, worked like this:

return snapshot.docs.map((doc) {
  return Todo(
    // before
    title: doc.data()['title'],
    // after
    title: (doc.data() as dynamic)['title'],
  );
}).toList();

In pubspec.yaml:

environment:
  sdk: ">=2.12.0 <3.0.0"
...
cloud_firestore: ^2.3.0
firebase_core: ^1.3.0

Firestore's data used to be a property of QueryDocumentSnapshot, but now it is a function, data().

And, as the error message suggests, what you are dealing with is indeed a Map<String, dynamic> Function(), i.e. a function that returns a map.

So, simply add empty parentheses to call the function data:

doc.data()['name']