Safely unpacking JSON objects in Dart

Unpacking data the easy way with pattern matching

Updated Article

Updated July 4, 2023, for Dart 3.

My original article described the complexities of unpacking data from a JSON object. If you scroll down, you can still read that article. However, pattern matching in Dart 3 has greatly simplified the process.

The problem

You can unpack a raw JSON string using the dart:convert library like so:

import 'dart:convert';

void main() {
  String rawJson = '{"name":"Mary","age":30}';
  dynamic map = jsonDecode(rawJson);
  String name = map['name'] as String;
  int age = map['age'] as int;
  Person person = Person(name, age);
}

class Person {
  Person(this.name, this.age);
  final String name;
  final int age;
}

However, the code above makes a lot of assumptions while getting from jsonDecode to the Person object:

• It assumes that map is a Map and not a List or a String.
• It assumes the map has String keys.
• It assumes that the map has a string key named name.
• It assumes that the map has a string key named age.
• It assumes the value of map['name'] is a String.