safe_json_decode

A drop-in replacement for jsonDecode that converts all JSON integers to doubles, preventing TypeError: type 'int' is not a subtype of type 'double'.

The problem

JSON (RFC 8259) makes no distinction between 5 and 5.0 — both are just "number". Dart's jsonDecode preserves the int/double distinction based on notation, which causes runtime crashes when a backend returns 29 for a field your model expects as double.

// Standard jsonDecode — crashes at runtime
final data = jsonDecode('{"price": 29}');
final double price = data['price'] as double; // TypeError!

This is especially common with REST APIs where the same field may return 29 or 29.0 depending on the value.

How other languages handle this

Language	jsonDecode('5') type	Cast to double needed?
Python	int	No (int + float works)
JavaScript	number	No (single type)
Go	float64	No (default)
Swift	Double	No (Codable)
Dart	int	Yes — crashes

Usage

import 'package:safe_json_decode/safe_json_decode.dart';

// Instead of: jsonDecode(response.body)
final data = safeJsonDecode(response.body);
final double price = data['price'] as double; // works!

API

safeJsonDecode(String source, {reviver})

Drop-in replacement for jsonDecode. All integer values in the JSON are converted to doubles. The optional reviver is called after the int-to-double conversion.

SafeJsonCodec

A Codec<Object?, String> for use with streaming APIs or anywhere a codec instance is needed.

const codec = SafeJsonCodec();
final data = codec.decode('{"price": 29}');
print(data['price'] is double); // true

SafeJsonDecoder

A Converter<String, Object?> that decodes JSON with int-to-double conversion. Note: chunked conversion via startChunkedConversion is not supported — use convert() directly or safeJsonDecode() for single-string inputs.

Why this exists

This package was created after the Dart team declined RFC #62776, a proposal to add a JsonNumericMode option to the SDK's jsonDecode. The team considers the int/double distinction a deliberate design choice (see sdk#55499).

While code generation tools like json_serializable and freezed can handle this, many projects use manual JSON parsing or need a simple fix without adding a build step.

License

BSD 3-Clause — see LICENSE.