JSON Parsing in Dart

What is JSON?

JSON (JavaScript Object Notation) is the standard format for data exchange between clients and servers. Dart's dart:convert library provides jsonDecode and jsonEncode for parsing and producing JSON.

Decoding JSON

import 'dart:convert';

void main() {
  // JSON string to Dart object:
  String jsonStr = '{"name":"Alice","age":30,"active":true}';
  Map<String, dynamic> user = jsonDecode(jsonStr);
  print(user['name']);    // Alice
  print(user['age']);     // 30

  // JSON array:
  String jsonArray = '[1, 2, 3, 4, 5]';
  List<dynamic> numbers = jsonDecode(jsonArray);
  print(numbers.cast<int>());  // [1, 2, 3, 4, 5]

  // Complex nested JSON:
  String complex = '''
  {
    "users": [
      {"id": 1, "name": "Alice", "roles": ["admin", "user"]},
      {"id": 2, "name": "Bob",   "roles": ["user"]}
    ],
    "total": 2
  }
  ''';
  var data = jsonDecode(complex) as Map<String, dynamic>;
  var users = data['users'] as List;
  for (var u in users) {
    print('$${u['name']}: $${u['roles'].join(', ')}');
  }
}

Encoding to JSON

import 'dart:convert';

void main() {
  var user = {
    'name': 'Bob',
    'age': 25,
    'scores': [98, 87, 92],
    'address': {'city': 'Phnom Penh', 'country': 'Cambodia'}
  };

  // Compact JSON:
  String compact = jsonEncode(user);
  print(compact);

  // Pretty-printed JSON:
  var encoder = JsonEncoder.withIndent('  ');
  print(encoder.convert(user));
}

JSON to Model Classes

class Product {
  final int id;
  final String name;
  final double price;
  final bool inStock;
  final List<String> tags;

  Product({
    required this.id,
    required this.name,
    required this.price,
    required this.inStock,
    required this.tags,
  });

  factory Product.fromJson(Map<String, dynamic> json) {
    return Product(
      id: json['id'] as int,
      name: json['name'] as String,
      price: (json['price'] as num).toDouble(),
      inStock: json['in_stock'] as bool,
      tags: List<String>.from(json['tags'] ?? []),
    );
  }

  Map<String, dynamic> toJson() => {
    'id': id,
    'name': name,
    'price': price,
    'in_stock': inStock,
    'tags': tags,
  };

  @override String toString() => 'Product($id: $name @ \$$price)';
}

void main() {
  var json = '{"id":1,"name":"Dart Book","price":29.99,"in_stock":true,"tags":["programming","dart"]}';
  var product = Product.fromJson(jsonDecode(json));
  print(product);  // Product(1: Dart Book @ $29.99)
  print(jsonEncode(product.toJson()));
}

Nested JSON Models

class Address {
  final String city, country;
  Address({required this.city, required this.country});
  factory Address.fromJson(Map<String, dynamic> j) =>
      Address(city: j['city'], country: j['country']);
  Map<String, dynamic> toJson() => {'city': city, 'country': country};
}

class User {
  final String name;
  final Address address;
  final List<Product> cart;

  User({required this.name, required this.address, required this.cart});

  factory User.fromJson(Map<String, dynamic> j) => User(
    name: j['name'],
    address: Address.fromJson(j['address']),
    cart: (j['cart'] as List).map((p) => Product.fromJson(p)).toList(),
  );
}

🧠 Quiz

Summary

Use jsonDecode(String) to parse JSON into Dart maps/lists. Use jsonEncode(Object) to convert Dart objects to JSON strings. Create model classes with fromJson factory constructors and toJson() methods for type-safe JSON handling. Use JsonEncoder.withIndent for pretty-printed output.