How to deep copy a Map/List in Dart

Using json.decode() and json.encode()

This approach works in any scenario (nested lists, nested maps…). You can actually clone multi-dimensional lists and maps without references.
Syntax:

List newList = json.decode(json.encode(oldList));
Map newMap = json.decode(json.encode(oldMap));

Example:

import 'dart:convert';
void main(){
  // Define a multi-dimensional map
  final Map oldMap = {
    "name" : {
      "first": "Joh",
      "last": "Doe"
    },
    "asset" : {
      "money" : {
        "bank": 1000,
        "cash": 100
      },
      "house": 1
    }
  };
  final Map newMap = json.decode(json.encode(oldMap));
  newMap["name"]["first"] = "Jesse";
  newMap["name"]["last"] = "Pinkman"; 
  newMap["asset"]["money"]["cash"] = 0; 
  print('oldMap: $oldMap');
  print('newMap: $newMap');
}

Output:

oldMap: {
  name: {first: Joh, last: Doe}, 
  asset: {
      money: {bank: 1000, cash: 100}, 
      house: 1
   }
}
newMap: {
  name: {first: Jesse, last: Pinkman}, 
  asset: {
      money: {bank: 1000, cash: 0}, 
      house: 1
   }
}

Using the spread syntax

This approach is quick and convenient for one-dimensional lists and maps. :::success Note: This method works with a one-dimensional List or Map. For cloning a multi-dimensional (nested) List or Map, use the first method. :::

Syntax:

List newList = [...oldList];
Map newMap = {...oldMap}

Example:

void main(){
  // List
  final List myList = ['A', 'B', 'C', 'D'];
  final List clonedList = [...myList];
  clonedList[0] = 'Dog';
  print('myList: $myList');
  print('clonedList: $clonedList');
  // Map
  final Map myMap = {
    'name': 'John',
    'age': 37
  };
  final Map clonedMap = {...myMap};
  clonedMap["name"] = "Marry";
  clonedMap['age'] = 4; 
  print('myMap: $myMap');
  print('clonedMap: $clonedMap');
}

Output:

myList: [A, B, C, D]
clonedList: [Dog, B, C, D]
myMap: {name: John, age: 37}
clonedMap: {name: Marry, age: 4}

Using the from() method

As the second approach, this one is quick and good for one-dimensional lists and maps. :::success Note: This method works with a one-dimensional List or Map. For cloning a multi-dimensional (nested) List or Map, use the first method. :::

Syntax:

List newList = List.from(oldList);
Map newMap = Map.from(oldMap);

Using []..addAll()

This approach is quick and good for only one-dimensional lists.

Syntax:

List newList  = []..addAll(oldList);

Example:

void main(){
 List oldList = [1, 2, 3];
 List newList  = []..addAll(oldList);
 newList[2] = 100;
 print('oldList: $oldList');
  print('newList: $newList');
}

Output:

oldList: [1, 2, 3]
newList: [1, 2, 100]