JSON in Dart & Flutter — jsonDecode, jsonEncode & Null Safety

Dart's dart:convert library provides jsonDecode() and jsonEncode() for JSON parsing. For Flutter apps, the community pattern is to add fromJson/toJson factory methods to your model classes, with optional code generation via json_serializable.

Parse JSON with jsonDecode

jsonDecode() from dart:convert parses a JSON string into a dynamic Dart object — a Map<String, dynamic> for objects, or a List<dynamic> for arrays.

import 'dart:convert';

void main() {
  final String jsonString = '{"id":1,"name":"Alice","email":"alice@example.com"}';
  final Map<String, dynamic> data = jsonDecode(jsonString);

  print(data['name']);  // Alice
  print(data['id']);    // 1
}

Encode to JSON with jsonEncode

jsonEncode() converts a Dart Map, List, String, int, double, bool, or null to a JSON string.

import 'dart:convert';

void main() {
  final data = {
    'name': 'Alice',
    'scores': [95, 87, 92],
    'active': true,
  };

  final String json = jsonEncode(data);
  print(json);
  // {"name":"Alice","scores":[95,87,92],"active":true}
}

Model class with fromJson and toJson

The standard Flutter pattern is to add a named constructor fromJson(Map<String, dynamic>) and a toJson() method to your model class.

class User {
  final int id;
  final String name;
  final String email;

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

  factory User.fromJson(Map<String, dynamic> json) {
    return User(
      id:    json['id']    as int,
      name:  json['name']  as String,
      email: json['email'] as String,
    );
  }

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

// Usage
final user = User.fromJson(jsonDecode(jsonString));
final backToJson = jsonEncode(user.toJson());

Handle null safety

With Dart null safety, use nullable types and the ?? operator to provide defaults for missing or null JSON fields.

class Profile {
  final int id;
  final String? bio;      // nullable — may be absent
  final String website;   // non-nullable — provide a default

  Profile({required this.id, this.bio, required this.website});

  factory Profile.fromJson(Map<String, dynamic> json) {
    return Profile(
      id:      json['id']      as int,
      bio:     json['bio']     as String?,
      website: json['website'] as String? ?? '',
    );
  }
}

Decode a JSON list (Flutter API response)

A common Flutter pattern for decoding a JSON array from an HTTP response.

import 'dart:convert';
import 'package:http/http.dart' as http;

Future<List<User>> fetchUsers() async {
  final response = await http.get(Uri.parse('https://api.example.com/users'));
  if (response.statusCode == 200) {
    final List<dynamic> list = jsonDecode(response.body);
    return list.map((json) => User.fromJson(json)).toList();
  }
  throw Exception('Failed to load users');
}

JSON Formatter →JSON to TypeScript →JSON to JSON Schema →JSON to CSV →

Frequently Asked Questions

How do I parse JSON in Dart?▾

Import dart:convert and call jsonDecode(jsonString). It returns a dynamic value — cast to Map<String, dynamic> for a JSON object or List<dynamic> for a JSON array.

How do I convert a Dart object to JSON?▾

Call jsonEncode(value) from dart:convert. The value must be a Map, List, String, int, double, bool, or null. For custom classes, implement a toJson() method that returns a Map<String, dynamic> and pass the result to jsonEncode.

How do I handle missing or null JSON fields in Dart with null safety?▾

Declare the property as a nullable type (String?, int?). In fromJson, cast with as String? and use ?? to provide a fallback: json['key'] as String? ?? 'default'.

What is json_serializable in Flutter?▾

json_serializable is a code-generation package that auto-generates fromJson/toJson methods. Annotate your class with @JsonSerializable() and run dart run build_runner build to generate the boilerplate.

How do I decode a JSON array from a Flutter HTTP response?▾

After getting the response body, call jsonDecode(response.body) which returns a List<dynamic>. Then map over it: (list as List).map((e) => MyModel.fromJson(e)).toList().

JSON in Other Languages

JSON in Python →JSON in JavaScript →JSON in TypeScript →JSON in Go →JSON in Java →JSON in PHP →JSON in Ruby →JSON in C# →JSON in Rust →JSON in Kotlin →JSON in Swift →JSON in Scala →

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