Built-in Subclasses

You can subclass built-in containers like Array, Map, Set, Uint8Array, and the other typed arrays. How they serialize depends on whether you decorate the subclass.

Undecorated: keeps built-in behavior

An undecorated subclass serializes and deserializes exactly like its base type — a named Uint8Array is still a JSON array of bytes.

import { JSON } from "json-as";

class MyBytes extends Uint8Array {
  constructor(length: i32 = 0) {
    super(length);
  }
}

const mb = new MyBytes(3);
mb[0] = 1;
mb[1] = 2;
mb[2] = 3;

JSON.stringify<MyBytes>(mb); // '[1,2,3]'
JSON.stringify(JSON.parse<MyBytes>("[4,5,6]")); // '[4,5,6]'

This is the right choice when you just want a named or extended container that still reads/writes as the underlying type.

Decorated: treated as a generated struct

Adding @json tells the transform to treat the subclass like any other generated class — it emits __SERIALIZE / __DESERIALIZE over the class's fields. For a built-in container that means its internal fields become the schema:

@json
class GenBytes extends Uint8Array {
  constructor(length: i32 = 0) {
    super(length);
  }
}

// Serializes the inherited internals, not the bytes:
// {"buffer":[…],"dataStart":<pointer>,"byteLength":3}
JSON.stringify<GenBytes>(new GenBytes(3));

That's almost never what you want on its own. So a decorated built-in subclass is really only useful together with a custom format:

@json
class HexBytes extends Uint8Array {
  constructor(length: i32 = 0) {
    super(length);
  }

  @serializer("string")
  serializer(self: HexBytes): string {
    /* encode bytes as a hex string */
  }

  @deserializer("string")
  deserializer(data: string): HexBytes {
    /* decode hex string back to bytes */
  }
}

See Custom Serialization for the full pattern.

Rule of thumb

• Want the built-in's JSON form? Don't decorate the subclass.
• Want a different wire format? Decorate it and add @serializer/@deserializer.
• Decorating without a custom format exposes the base type's internal fields — mark any you don't want with @omit, or (better) supply a custom format.