Safely unpacking JSON objects in Dart

analysis_options.yaml

include: package:pedantic/analysis_options.yaml

analyzer:
  strong-mode:
    implicit-casts: false
    implicit-dynamic: false

main.dart

import 'dart:convert';

void main() {
  implicitCastsExample();
  implicitDynamicExample();
  jsonMapExample();
  jsonListExample();
}

/// Implicit casts example

void implicitCastsExample() {
  // num x = 3;
  // int y = x;

  num x = 3;
  int y = x as int;

  // num x = 3.1;
  // int y = x as int;
}

/// Implicit dynamic example

void implicitDynamicExample() {
  // var x;
  dynamic x;
  x = 3;
  // x.length;
}

/// Map example

String jsonMapString = '''
{
  "name":"mary",
  "age":35
}
''';

void jsonMapExample() {
  try {
    dynamic myMap = json.decode(jsonMapString);
    if (myMap is! Map<String, dynamic>) throw FormatException();
    final person = Person.fromJson(myMap);
    print('${person.name} ${person.age}');
  } catch (error) {
    print('JSON is in the wrong format');
  }
}

/// List example

String jsonListString = '''
[
  {
    "name":"bob",
    "age":22
  },
  {
    "name":"mary",
    "age":35
  }
]
''';

void jsonListExample() {
  final myList = <Person>[];
  try {
    dynamic jsonList = json.decode(jsonListString);
    if (jsonList is! List) throw FormatException();
    for (dynamic item in jsonList) {
      if (item is! Map<String, dynamic>) continue;
      final person = Person.fromJson(item);
      myList.add(person);
    }
  } on FormatException {
    print('JSON is in the wrong format');
  }

  myList.forEach((person) => print('${person.name} ${person.age}'));
}

/// Data class

class Person {
  final String name;
  final int age;

  Person(this.name, this.age);

  factory Person.fromJson(Map<String, dynamic> json) {
    dynamic name = json['name'];
    dynamic age = json['age'];
    if (name is! String) name = '';
    if (age is! int) age = 0;
    return Person(name, age);
  }
}

pubspec.yaml

name: dart_playground
description: A simple command-line application.
version: 1.0.0

environment:
  sdk: '>=2.12.0 <3.0.0'

dev_dependencies:
  pedantic: ^1.10.0