Convert JSON to Dart
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Converting JSON to Dart classes

Dart classes are the standard way to handle JSON in Flutter applications. Unlike JavaScript, Dart requires explicit typed fields. This tool generates Dart class definitions including a fromJson factory constructor for deserialization and a toJson method for encoding, working directly with dart:convert.

The converter follows Dart conventions: PascalCase class names, camelCase fields, List<T> for arrays, and nullable types (String?) for optional fields. Each class includes fromJson/toJson methods with proper null-safety annotations. The output is compatible with manual parsing and code-generation tools like json_serializable.

{
  "name": "Alice",
  "age": 30,
  "roles": ["admin"],
  "bio": null
}

class Root {
  final String name;
  final int age;
  final List<String> roles;
  final String? bio;

  Root({required this.name,
    required this.age,
    required this.roles, this.bio});

  factory Root.fromJson(
    Map<String, dynamic> json) =>
    Root(
      name: json['name'],
      age: json['age'],
      roles: List<String>.from(
        json['roles']),
      bio: json['bio'],
    );
}

Get the most out of this tool

• Use factory MyClass.fromJson(Map<String, dynamic>) for manual JSON parsing in Flutter apps.
• For large projects, use generated classes as a starting point and add json_serializable annotations.
• Mark fields as nullable (String?) when the API may return null to prevent runtime type errors in Flutter.

Dart model classes for Flutter JSON

Dart is the language behind Flutter, Google's cross-platform UI framework. When Flutter apps communicate with REST APIs, they receive JSON responses that must be parsed and mapped to Dart objects. Dart's dart:convert library provides jsonDecode() which returns dynamic — a type that carries no compile-time type information. To get type-safe access to JSON data, you need Dart model classes with fromJson() constructors. This converter generates those classes automatically from your JSON input.

The converter generates Dart classes with standard conventions: fields use camelCase, class names use PascalCase, and each class includes a fromJson(Map<String, dynamic> json) factory constructor and a toJson() Map<String, dynamic> method. These two methods are the standard interface for JSON serialization in Dart without code generation. The factory constructor pattern is preferred over a regular constructor because it allows returning different instances or handling errors during construction.

JSON type mapping in Dart follows straightforward rules. JSON strings become String. JSON integers become int. JSON floating-point numbers become double. JSON booleans become bool. JSON null maps to nullable types in Dart's null-safe type system (String?, int?, etc.). JSON arrays become List<T>. Nested JSON objects become nested Dart class instances, with the fromJson constructor calling the nested class's fromJson recursively.

Dart's null safety, introduced in Dart 2.12 and required in Flutter 2.0+, means every type is non-nullable by default. A field of type String cannot be null — if you try to assign null to it, Dart throws a compile-time error. When a JSON field might be absent or null in the response, the Dart field must be declared as String? (nullable). The converter handles this by marking fields nullable when their JSON values are null in the provided sample — review the output based on your API's documented contract.

The json_serializable package (with build_runner) offers an alternative to hand-written fromJson/toJson methods. With @JsonSerializable() annotations and running flutter pub run build_runner build, the boilerplate is generated from annotated class fields. The downside is the added build step and dependency. For simple models with a few fields, hand-written fromJson/toJson as generated by this tool are often simpler and more transparent.

For Flutter apps using state management with Riverpod, BLoC, or Provider, Dart model classes are the natural data layer. Immutable model classes — declared with final fields and no setters — work well with these patterns. The copyWith() method, which returns a new instance with some fields changed, is commonly generated alongside fromJson/toJson. Freezed is a popular code generation package that produces immutable models with copyWith, equality, and JSON support in one step.

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