JSON to Dart Class — fromJson / toJson, null-safe, Flutter-ready

Real-world use cases

• Model a Dio response without hand-writing fromJson

  You hit an endpoint with Dio and want a typed model instead of digging through `response.data['user']['profile']['avatar']` with stringly-typed keys. Paste one sample response, set the root class name, and copy the generated Dart. You get `final` fields, a `factory User.fromJson(Map<String, dynamic>)`, and `toJson()` for the request body — nested objects already lifted into their own classes and wired up. What used to be 15 minutes of typing `as String` casts becomes a paste-and-copy, and the camelCase rename happens automatically so `created_at` reads as `createdAt` in your widget tree while still serialising back to the wire key.

• Generate json_serializable models for a build_runner project

  Your Flutter app already runs build_runner with json_serializable, and you just got a new payload from the backend team. Flip on the json_serializable option, paste the JSON, and the tool emits the `@JsonSerializable()` class, the `part 'user.g.dart';` directive, `@JsonKey(name: 'created_at')` for every renamed field, and the `_$UserFromJson` / `_$UserToJson` bridges. Drop the file in, run `dart run build_runner build`, and the generated code matches the conventions your codebase already uses — no second pass to add annotations by hand.

• Turn a Firebase document into an immutable DTO

  Firestore hands you a `Map<String, dynamic>` with mixed types and optional fields. Paste a representative document and keep `final` on so the model is immutable — the Flutter-recommended default for state that flows through your widgets. Fields that some documents omit come out as nullable `T?`, which matches Firestore's reality where not every doc has every key, and the null-guarded fromJson (`json['x'] == null ? null : ...`) means a missing field won't throw at parse time.

• Match a Supabase row to a strongly-typed class

  A Supabase select returns snake_case columns (`user_id`, `is_active`, `last_seen`). Paste a row and the tool camelCases the Dart field names (`userId`, `isActive`, `lastSeen`) while keeping `fromJson`/`toJson` bound to the original column names, so the model round-trips against the database without a custom converter. Numbers that are sometimes integer (a count) and sometimes decimal (an average) widen to `num`, so you don't get a runtime cast error the first time the column returns a float.

• Stop runtime type errors from untyped Map access

  Reaching into `data['items'][0]['price']` works until the day the backend sends a string where you expected a number and your app crashes in production. Generate a real class with this tool, parse once through `fromJson`, and every downstream access is statically typed — the analyzer flags a mismatch at compile time instead of your users hitting a red screen. Arrays of objects fold into one class, so a list of 200 items still gives you exactly one typed element model to reason about.

Common pitfalls

• Treating every number as int. JSON has one number type, so a field that is 0 in your sample but actually carries cents or an average will be inferred as int and silently truncate a later 4.5 to a cast error. This tool widens int/double unions to num — but if a field is only ever whole in your sample yet can hold decimals in production, change it to double or num by hand before shipping.

• Forgetting build_runner in json_serializable mode. The @JsonSerializable() annotation, the part directive, and the _$ClassFromJson/_$ClassToJson references all need `dart run build_runner build` to generate the .g.dart file. Add json_annotation + json_serializable + build_runner to pubspec.yaml and run codegen — or leave json_serializable off for a self-contained file with hand-written methods.

• Dropping the wire key when renaming. fromJson reads json['site_admin'] because that is the JSON key, even though the Dart field is siteAdmin. Change the map key to 'siteAdmin' too and deserialisation looks for a key the server never sends, leaving the field null. Keep the original key in fromJson/toJson (or @JsonKey(name:) in codegen mode).

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