Insert map key as an id inside map value - flutter

Question

My goal is to insert a map key as an id inside map value (which is also a map, the nested one).
Say I have a map as follows
Map<String, dynamic> userMap = {
"id_1" : {
"name": "John",
"surname": "Doe",
"age": 25,
},
"id_2" : {
"name": "Jeremy",
"surname": "Smith",
"age": 42,
}
};
I want to convert this to
Map<String, dynamic> userMap = {
"id_1" : {
"id": "id_1",
"name": "John",
"surname": "Doe",
"age": 25,
},
"id_2" : {
"id": "id_2",
"name": "Jeremy",
"surname": "Smith",
"age": 42,
}
};
I tried this
userMap.entries.map((entry) => (entry.value as Map).putIfAbsent('id', () => entry.key));
but it is not working. Instead it is giving result as (id_1, id_2). What I am doing wrong ?? Is there any way to achieve the desired one ??

Answer 1

userMap.entries.map((entry) => (entry.value as Map).putIfAbsent('id', () => entry.key));
The reason why that doesn't work is because you don't do anything with the result of calling .map(). From the Iterable.map documentation:
Returns a new lazy Iterable.... As long as the returned Iterable is not iterated over, the supplied function f will not be invoked.
Your callback function thus is never called and will not mutate the original Map. You could forcibly iterate over the result of .map() (e.g. userMap.entries.map(...).toList()) to get your desired result. However that's wasteful and misuses Iterable.map.
Iterable.map is meant to be used to perform a functional transformation on an Iterable; you should not use it when the callback has side effects. (A "side effect" is something that the callback does other than just returning a value.) For callbacks where you care about side effects, either use Iterable.forEach or use a normal for loop.

Answer 2

You can use this:
userMap.forEach((k,v) => {(v as Map).putIfAbsent('id', () => k)});