Dart CI codecov Pub

A simple and lightweight JSON data container for Dart and Flutter.


The Dart language doesn't have an official way to mark an object as a piece of JSON data. You have to carry it in a simple string and then use dart:convert when you need the decoded value. Furthermore, since JsonCodec is designed for general purpose, encoding and decoding operations result completely type-unsafe. json_string not only keeps your JSON data as light as possible, but also encapsulates dart:convert directly into it, offering you better soundness and a much clearer syntax.


Add json_string to pubspec.yaml under the dependencies subsection.

  json_string: ^3.0.1


Install packages from the command line (or using your favourite IDE).

with pub:

$ pub get

with flutter:

$ flutter pub get


In your library, add the following line:

import 'package:json_string/json_string.dart';

How to use it

All you need is the JsonString class. This is where your JSON data is stored. When you create a new instance, your JSON data is evaluated and minified.


Check if your string represents a valid JSON, simply using the default constructor:

try {
    final jsonString = JsonString('{"username":"john_doe","password":"querty"}');
    // ...
} on JsonFormatException catch (e) {
    print('Invalid JSON format: $e');

Or just work with null values, if you prefer:

final jsonString = JsonString.orNull('{"username":"john_doe","password":"querty"}');

if (jsonString == null) {
    print('Invalid JSON format');
// ...

Then, access your data through the source property:

final source = jsonString.source;

print(source); // {"username":"john_doe","password":"querty"}

Or read the equivalent decodedValue object, without using any other library:

final credentials = jsonString.decodedValue;

print(credentials['username']); // john_doe
print(credentials['password']); // querty


JsonString provides a set of different methods to encode Dart types, all implemented in a type-safe way.

Plain objects

Mark as Jsonable your "json2dart" style class, so you won't forget to implement the toJson() method:

// class declaration
class User with Jsonable {
  String username;
  String password;

    required this.username,
    required this.password,
  Map<String, dynamic> toJson() => <String, dynamic>{
        'username': username,
        'password': password,

// encoding
final user = User(username: 'john_doe', password: 'qwerty');
final jsonString = JsonString.encodeObject(user);

Or, provide an encoder to specify how to map your object:

final jsonString = JsonString.encodeObject(user, encoder: (u) => {
    'ba': btoa("${u.username}:${u.password}"),

Object lists

In the same way, you can encode a list of objects:

final userList = [
    User(username: 'john_doe', password: 'qwerty'),
    User(username: 'clara_brothers', password: 'asdfgh')

// default encoding
final jsonString1 = JsonString.encodeObjectList(userList);

// custom encoding
final jsonString2 = JsonString.encodeObjectList(userList, encoder: (u) => {
    'ba': btoa("${u.username}:${u.password}"),

Primitive lists

If you want to encode a list of primitive values (int, double, String, bool or Null), use encodePrimitiveList():

// integers
final fibonacci = [1, 1, 2, 3, 5, 8, 13];
final jsonString = JsonString.encodePrimitiveList(fibonacci);

// strings
final message = ["h", "e", "l", "l", "o", "!"];
final jsonString = JsonString.encodePrimitiveList(message);

// doubles
final temperatures = [16.0, 18.0, 21.0, 19.0];
final jsonString = JsonString.encodePrimitiveList(temperatures);

// booleans
final flags = [false, false, true, false];
final jsonString = JsonString.encodePrimitiveList(flags);

// nulls
final usefulList = [null, null, null, null];
final jsonString = JsonString.encodePrimitiveList(usefulList);

Primitive values

Sometimes you have to encode just a single value, in that case use encodePrimitiveValue():

// integer
final answer = 42;
final jsonString = JsonString.encodePrimitiveValue(answer);

// string
final message = "hello!";
final jsonString = JsonString.encodePrimitiveValue(message);

// double
final pi = 3.14159;
final jsonString = JsonString.encodePrimitiveValue(pi);

// boolean
final amIaGenius = false;
final jsonString = JsonString.encodePrimitiveValue(amIaGenius);

// null
final usefulValue = null;
final jsonString = JsonString.encodePrimitiveValue(usefulValue);


It's time to access the Dart counterpart of some JSON data. Here json_string helps you with several solutions. You have properties for simple general access and methods for some finer decoding operations.


When you construct a JsonString object, it checks on your behalf if the source represents a valid piece of JSON data, but it doesn’t tell you what kind of data it contains. If you don’t know the expected type or you simply don’t care about it, just access the decodedValue property. It works every time:

// any type
final decodedValue = jsonString.decodedValue;

But since most of the time you need to cope with big objects or lists, if you have this information before, it is better to use decodedValueAsMap or decodedValueAsList:

// object expected
try {
    final decodedObject = jsonString.decodedValueAsMap;
    print(decodedObject.runtimeType); // Map<String, dynamic>
} on JsonDecodingError catch (e) {
    throw "This is not an object.";

// list expected
try {
    final decodedList = jsonString.decodedValueAsList;
    print(decodedList.runtimeType); // List<dynamic>
} on JsonDecodingError catch (e) {
    throw "This is not a list.";


Decoding methods are similar to the encoding ones. They simply do the job the other way round.

Plain objects and object lists (non-nullable)
// class declaration
class User {
  String username;
  String password;

    required this.username,
    required this.password,

  static User fromJson(Map<String, dynamic> json) => User(
        username: json['username'] as String,
        password: json['password'] as String,

// object decoding
final jsonString = JsonString('{"username":"john_doe","password":"querty"}');

final user = jsonString.decodeAsObject(User.fromJson);

// object list decoding
final jsonString = JsonString('''[

final userList = jsonString.decodeAsObjectList(User.fromJson);
Plain objects and object lists (nullable)
// class declaration
class User {
  String? username;
  String? password;


  static User fromJson(Map<String, dynamic>? json) => User(
        username: json?['username'] as String?,
        password: json?['password'] as String?,

// object decoding
final jsonString = JsonString('null');

final user = jsonString.decodeAsNullableObject(User.fromJson);

// object list decoding
final jsonString = JsonString('''[

final userList = jsonString.decodeAsNullableObjectList(User.fromJson);
Primitive lists
// integer list decoding
final jsonString = JsonString('[1, 1, 2, 3, 5, 8, 13]');
final fibonacci = jsonString.decodeAsPrimitiveList<int>();

// string list decoding
final jsonString = JsonString('["h", "e", "l", "l", "o", "!"]');
final message = jsonString.decodeAsPrimitiveList<String>();

// double list decoding
final jsonString = JsonString('[16.0, 18.0, 21.0, 19.0]');
final temperatures = jsonString.decodeAsPrimitiveList<double>();

// boolean list decoding
final jsonString = JsonString('[false, false, true, false]');
final flags = jsonString.decodeAsPrimitiveList<bool>();
Primitive values
// integer value decoding
final jsonString = JsonString('42');
final answer = jsonString.decodeAsPrimitiveValue<int>();

// string value decoding
final jsonString = JsonString('hello!');
final message = jsonString.decodeAsPrimitiveValue<String>();

// double value decoding
final jsonString = JsonString('3.14159');
final pi = jsonString.decodeAsPrimitiveValue<double>();

// boolean value decoding
final jsonString = JsonString('false');
final amIaGenius = jsonString.decodeAsPrimitiveValue<bool>();
Nullable primitive lists and values
// nullable integer list decoding
final jsonString = JsonString('[42, null, 25, 74, null]');
final availableAges = jsonString.decodeAsPrimitiveList<int?>();

// nullable integer value decoding
final jsonString = JsonString('null');
final maybeAnswer = jsonString.decodeAsPrimitiveValue<int?>();


Here's a list of some advanced available options.

Complex encoding

If you need to encode complicated structures, you can use JsonString.encode() which is just a wrapper around the built-in dart:convert encode method:

const data = [{
  "key0": [1, 2, 3],
  "key1": 123,
  "key2": "123",
}, "value1", false];
try {
  final jsonString = JsonString.encode(data);
  // ...
} on JsonEncodingError catch (e) {
  print('${data.toString()} is impossible to encode : $e');


Decoding operations may take time and be expensive in terms of computing resources. When you construct a JsonString object, you can specify an enableCache flag, which keeps a copy of the decodedValue property. This makes every usage of decoding properties or methods way faster. The only drawback to this is the necessary memory occupation:

final jsonString = JsonString(source, enableCache: true);
final decodedMap = jsonString.decodedValueAsMap; // immediate access

Turn off Jsonable check

If you don't want to use Jsonable, you can set checkIfJsonable to false when encoding objects.

// singleObject is not a Jsonable object
final jsonString = JsonString.encodeObject(singleObject, checkIfJsonable: false);
// objectList is not a list of Jsonable objects
final jsonString = JsonString.encodeObjectList(objectList, checkIfJsonable: false);


If you find a bug, or you would like to see a new feature, please create an issue.


A simple and lightweight JSON data container for Dart and Flutter.