Duplicate map is being added to list - dart

Question

void main() {
Map firstMap = { "id": 1, "value": [{"key": "A"}, {"key": "B"}]};
List<Map> newList = [];
List valueList = firstMap["value"];
for(int y=0; y<valueList.length; y++) {
Map newMap = firstMap;
newMap["value"] = [{"key": valueList[y]["key"]}];
newList.add(newMap);
}
print(newList);
}
My expectation for newList is to look like:
[{id: 1, value: [{key: A}]}, {id: 1, value: [{key: B}]}]
but the actual value of newList is
[{id: 1, value: [{key: B}]}, {id: 1, value: [{key: B}]}]
I am not sure what to try here.
valueList[y]["key"] seems to have the correct value in each iteration.
The code above has been shared as a link here to dart pad.
How would I achieve the same? Also what is going on with my code, which is leading to such a result.
Open with Dart pad

Answer 1

Error that you are doing is as already mentioned in the comment, reassignment to different variable does not make a copy of data:
Map a = {"a": 1}
Map b = a; // b is just one more reference to same object
b["a"] = 2;
print(a); // -> {"a": 2}
Transformations from one shape to another can be conveniently done with map()
https://dartpad.dartlang.org/8283e91c2c201ddb65ef073e76995f1d
void main() {
Map firstMap = { "id": 1, "value": [{"key": "A"}, {"key": "B"}]};
List valueList = firstMap["value"];
List<Map> newList = valueList.map((valueItem) => {
"id": firstMap["id"],
"value": [valueItem]
}).toList();
print(newList);
}