'what is the reason for this error and how to solve it

add method produce error saya his expression has a type of 'void' so its value can't be used.

 Future addtocart({
    String? productID,
    BuildContext? context,
  }) async {
    List<String>? cartitems = ["gabbage"];
    await _firestore
        .collection("Users")
        .doc(_currentUser!.uid)
        .collection("Cart")
        .doc(_currentUser!.uid)
        .get()
        .then((doc) {
      if (doc.data()!["UserCart"].contains(productID)) {
        Fluttertoast.showToast(
          msg: 'Product is already in the cart',
          gravity: ToastGravity.CENTER,
        );
      } else {
        _firestore
            .collection("Users")
            .doc(_currentUser!.uid)
            .collection("Cart")
            .doc(_currentUser!.uid)
            .update({"UserCart": cartitems.add(productID!)});
      }
    });
  }
}

flutter

Solution 1:^[1]

Instead of cartitems.add(productID!), you can do FieldValue.arrayUnion(productID!). You can find the documentation of FieldView here: https://pub.dev/documentation/cloud_firestore/latest/cloud_firestore/FieldValue-class.html

Solution 2:^[2]

cartitems.add(productID!);

It's not returning anything so how can you assign it to a map

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution	Source
Solution 1
Solution 2	Kai - Kazuya Ito

'what is the reason for this error and how to solve it

Solution 1:[1]

Solution 2:[2]

Sources

Related Questions

Solution 1:^[1]

Solution 2:^[2]