json-as

Fast, type-safe JSON for AssemblyScript.

GitHub · npm — npm install json-as

json-as is a compiler transform: mark a class @json and it generates specialized serialize/deserialize code at build time. No runtime reflection, no hand-written schemas — just JSON.parse<T> and JSON.stringify, reaching multi-GB/s throughput with SIMD/SWAR scanning.

import { JSON } from "json-as";

@json
class Vec3 {
  x: f32 = 0;
  y: f32 = 0;
  z: f32 = 0;
}

const v = JSON.parse<Vec3>('{"x":1.5,"y":2.5,"z":3.5}');
JSON.stringify(v); // '{"x":1.5,"y":2.5,"z":3.5}'

Highlights

• Typed — JSON.parse<T> / JSON.stringify over your own @json classes, nested to any depth.
• Batteries included — strings, all number types, bool, Date, arrays, Map, Set, typed arrays, ArrayBuffer.
• Dynamic when you need it — JSON.Value, JSON.Obj, and JSON.Raw for unknown or open-ended shapes.
• Lazy fields — defer parsing of fields you rarely read; untouched fields pass through their raw bytes on serialize.
• Custom formats — @serializer / @deserializer for compact or bespoke wire formats.
• Fast — three interchangeable scanning modes (SWAR, SIMD, NAIVE) selected at build time.

Guide

• Getting Started — install, enable the transform, first round-trip
• Basic Usage — fields, nesting, aliases, optional fields, nullability
• Built-in Types — dates, collections, typed arrays
• Custom Serialization — @serializer / @deserializer
• Built-in Subclasses — subclassing Array, Map, typed arrays
• Lazy Fields — on-demand field parsing
• Performance — modes, the fast path, and benchmarks

Reference

• Decorators — every decorator in one place
• Dynamic Types — JSON.Value / JSON.Obj / JSON.Raw
• Configuration — build-time JSON_* environment variables
• Runtime Behavior — the shared buffer, JSON.internal, JSON.Memory

Deep Dive

How the interesting parts actually work:

• Generated Code & the Fast Path — what @json compiles to
• SWAR & SIMD Scanning — finding structure many lanes at a time
• The Lazy Slot — the packed u64 behind lazy fields
• The Serialization Buffer — zero-copy, zero-alloc round-trips

Testing your AssemblyScript too? See as-test.