What is the difference between jsonEncode vs jsonDecode ?

Overview

jsonEncode

jsonDecode

dart:convert

jsonEncode — Dart Object → JSON String

dartCopy
import 'dart:convert';

// Map to JSON string
final user = {'name': 'Alice', 'age': 28, 'isActive': true};
final jsonString = jsonEncode(user);
print(jsonString); // {"name":"Alice","age":28,"isActive":true}

// List to JSON string
final items = [1, 'hello', true, null];
print(jsonEncode(items)); // [1,"hello",true,null]

jsonDecode — JSON String → Dart Object

dartCopy
import 'dart:convert';

final jsonString = '{"name":"Alice","age":28,"isActive":true}';

// Returns dynamic — usually a Map or List
final Map<String, dynamic> user = jsonDecode(jsonString);
print(user['name']); // Alice
print(user['age']);  // 28

// Decoding a JSON array
final listString = '[1, 2, 3, 4]';
final List<dynamic> numbers = jsonDecode(listString);
print(numbers); // [1, 2, 3, 4]

With HTTP API Response

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

Future<User> fetchUser(int id) async {
  final response = await http.get(
    Uri.parse('https://api.example.com/users/$id'),
  );

  if (response.statusCode == 200) {
    // response.body is a String → decode it to Map
    final Map<String, dynamic> data = jsonDecode(response.body);
    return User.fromJson(data);
  }
  throw Exception('Failed to load user');
}

// Sending data — encode Dart map to JSON string
Future<void> createUser(User user) async {
  await http.post(
    Uri.parse('https://api.example.com/users'),
    headers: {'Content-Type': 'application/json'},
    body: jsonEncode(user.toJson()), // Map → JSON string for request body
  );
}

Comparison Table

jsonEncode

jsonDecode

String

dynamic

Custom Object Serialization

dartCopy
class User {
  final String name;
  final int age;

  User({required this.name, required this.age});

  // Dart object → Map (for jsonEncode)
  Map<String, dynamic> toJson() => {'name': name, 'age': age};

  // Map (from jsonDecode) → Dart object
  factory User.fromJson(Map<String, dynamic> json) =>
      User(name: json['name'], age: json['age']);
}

// Full round-trip
final user = User(name: 'Alice', age: 28);
final encoded = jsonEncode(user.toJson());   // '{"name":"Alice","age":28}'
final decoded = User.fromJson(jsonDecode(encoded)); // User object

Memory Aid:

• textCopy

  jsonEncode

  → Encode = Pack into a string (send it away)
• textCopy

  jsonDecode

  → Decode = Unpack from a string (read it in)