Dart JSON serialization quick reference
The three patterns you need for every Flutter project.
Object → JSON string
Implement Map<String, dynamic> toJson() on your class, then call jsonEncode(obj.toJson()) from dart:convert. The result is a String you can send as an HTTP body.
JSON string → Object
Call jsonDecode(jsonString) to get a Map<String, dynamic>, then pass it to your MyClass.fromJson(map) factory constructor. Use the JSON to Dart tool to generate fromJson.
With dio HTTP client
Pass model.toJson() directly as the data parameter in dio — dio.post('/endpoint', data: model.toJson()). Dio serializes Map<String, dynamic> automatically, no jsonEncode() needed.
With Firestore
Write: doc.set(model.toJson()). Read: MyClass.fromJson(snap.data()!). The toJson() map maps directly to Firestore document fields — no extra conversion layer needed.
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Dart JSON serialization — the most common questions.
How do I add a toJson method to a Dart class?
A toJson() method returns Map<String, dynamic> with each field mapped to its JSON key. For nested objects call their own toJson(). For lists of objects use .map((e) => e.toJson()).toList(). Paste your class above and JSONshift generates the complete implementation.
How do I convert a Dart object to a JSON string?
Two steps: implement toJson() returning Map<String, dynamic>, then call jsonEncode(obj.toJson()) from dart:convert. The result is a String you can use as an HTTP body, write to SharedPreferences, or log for debugging.
What is the difference between toJson() and toMap() in Dart?
They are functionally identical — both return Map<String, dynamic>. toJson() is the more common naming convention and aligns with json_serializable. toMap() is sometimes used when the map isn't specifically for JSON serialization, e.g. for Firestore or SQLite. Use whichever matches your team's convention.
How do I handle nullable fields in toJson with null safety?
Simply include the nullable field in the map — it will serialize as null in the JSON. For example: 'email': email. If you want to omit null fields from the JSON output entirely, use: if (email != null) 'email': email inside the map using a spread with a conditional, or filter the map after building it.
