JSON in Dart

import 'dart:convert';

class User {
  final int id;
  final String name;
  final String email;
  final bool active;
  final Address address;
  final List<String> tags;

  User({
    required this.id,
    required this.name,
    required this.email,
    required this.active,
    required this.address,
    required this.tags,
  });

  factory User.fromJson(Map<String, dynamic> json) => User(
    id: json['id'] as int,
    name: json['name'] as String,
    email: json['email'] as String,
    active: json['active'] as bool,
    address: Address.fromJson(json['address'] as Map<String, dynamic>),
    tags: (json['tags'] as List<dynamic>).map((e) => e as String).toList(),
  );

  Map<String, dynamic> toJson() => {
    'id': id,
    'name': name,
    'email': email,
    'active': active,
    'address': address.toJson(),
    'tags': tags,
  };
}

class Address {
  final String street;
  final String city;
  final String zip;

  Address({required this.street, required this.city, required this.zip});

  factory Address.fromJson(Map<String, dynamic> json) => Address(
    street: json['street'] as String,
    city: json['city'] as String,
    zip: json['zip'] as String,
  );

  Map<String, dynamic> toJson() => {'street': street, 'city': city, 'zip': zip};
}

void main() {
  final jsonStr = '{"id":1,"name":"Alice","email":"alice@example.com","active":true,"address":{"street":"123 Main","city":"Springfield","zip":"12345"},"tags":["admin","user"]}';
  final user = User.fromJson(jsonDecode(jsonStr));
  print(user.name); // -> Alice
  print(jsonEncode(user.toJson())); // -> round-trip back to JSON
}

// pubspec.yaml dependencies:
//   json_annotation: ^4.8.0
//   dev_dependencies:
//     build_runner: ^2.4.0
//     json_serializable: ^6.7.0

import 'package:json_annotation/json_annotation.dart';
part 'user.g.dart'; // generated by: dart run build_runner build

@JsonSerializable()
class User {
  final int id;
  final String name;
  final String email;
  @JsonKey(name: 'is_active')
  final bool isActive;
  final List<String> tags;

  User({
    required this.id,
    required this.name,
    required this.email,
    required this.isActive,
    required this.tags,
  });

  factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
  Map<String, dynamic> toJson() => _$UserToJson(this);
}

// Run code generation:
// dart run build_runner build --delete-conflicting-outputs

function jsonToDart(obj, name = "Root") {
  const classes = [];
  function infer(val, fieldName) {
    if (val === null) return "dynamic";
    if (typeof val === "string") return "String";
    if (typeof val === "number") return Number.isInteger(val) ? "int" : "double";
    if (typeof val === "boolean") return "bool";
    if (Array.isArray(val)) {
      const first = val.find(v => v !== null);
      if (!first) return "List<dynamic>";
      return `List<${infer(first, fieldName)}>`;
    }
    if (typeof val === "object") {
      const cls = fieldName.charAt(0).toUpperCase() + fieldName.slice(1);
      build(val, cls);
      return cls;
    }
    return "dynamic";
  }
  function build(obj, cls) {
    const fields = Object.entries(obj).map(([k, v]) =>
      `  final ${infer(v, k)} ${k};`
    );
    classes.push(`class ${cls} {\n${fields.join("\n")}\n}`);
  }
  build(obj, name);
  return classes.join("\n\n");
}

console.log(jsonToDart({ id: 1, name: "Alice", scores: [98, 85] }, "User"));
// class User {
//   final int id;
//   final String name;
//   final List<int> scores;
// }

import json

def json_to_dart(obj: dict, class_name: str = "Root") -> str:
    classes = []

    def infer(val, name):
        if val is None:
            return "dynamic"
        if isinstance(val, bool):
            return "bool"
        if isinstance(val, int):
            return "int"
        if isinstance(val, float):
            return "double"
        if isinstance(val, str):
            return "String"
        if isinstance(val, list):
            if not val:
                return "List<dynamic>"
            return f"List<{infer(val[0], name)}>"
        if isinstance(val, dict):
            cls = name[0].upper() + name[1:]
            build(val, cls)
            return cls
        return "dynamic"

    def build(obj, cls):
        fields = [f"  final {infer(v, k)} {k};" for k, v in obj.items()]
        classes.append(f"class {cls} {{\n" + "\n".join(fields) + "\n}")

    build(obj, class_name)
    return "\n\n".join(classes)

data = json.loads('{"id": 1, "name": "Alice", "active": true}')
print(json_to_dart(data, "User"))
# class User {
#   final int id;
#   final String name;
#   final bool active;
# }