SCALE Codecs for JavaScript and TypeScript

A TypeScript implementation of SCALE (Simple Concatenated Aggregate Little-Endian) transcoding (see Rust implementation here), which emphasizes JS-land representations and e2e type-safety. These types are described below.

⚠️ This TypeScript implementation of Parity’s SCALE Codecs is in beta. If you encounter a bug or want to give feedback on the API design, please create an issue.

Setup

If you’re using Deno, simply import via the denoland/x specifier.

import * as s from "https://deno.land/x/scale/mod.ts";

If you’re using Node, install as follows.

npm install parity-scale-codec

NOTE: The published package name is (while in beta) subject to change

Then import as follows.

import * as s from "parity-scale-codec";

Usage

Import the library
Define a codec via the library’s functions, whose names correspond to types
Utilize the codec you’ve defined

Example

import * as s from "https://deno.land/x/scale/mod.ts";

const codec = s.record(
  ["name", s.str],
  ["nickName", s.str],
  ["superPower", s.option(s.str)],
);

const valueToEncode = {
  name: "Magdalena",
  nickName: "Magz",
  superPower: "Hydrokinesis",
};

const encodedBytes = codec.encode(valueToEncode);
const decodedValue = codec.decode(encodedBytes);

assertEquals(decodedValue, valueToEncode);

To extract the JS-native TypeScript type from a given codec, use the Native utility type.

type NativeType = s.Native<typeof codec>;

assertTypeEquals<NativeType, {
  name: string;
  nickName: string;
  superPower: string | undefined;
}>();

In cases where codecs are exceptionally large, we may want to spare the TS checker of extra work.

interface Person {
  name: string;
  nickName: string;
  superPower: string | undefined;
}

const codec: Codec<Person> = s.record(
  ["name", s.str],
  ["nickName", s.str],
  ["superPower", s.option(s.str)],
);

This has the added benefit of producing type errors if the codec does not directly mirror the TS type.

const codec: Codec<NativeType> = s.record(
  //  ~~~~~
  //  ^ error (message below)
  ["nickName", s.str],
  ["superPower", s.option(s.str)],
);

Hovering over the error squigglies will reveal the following diagnostic.

Type 'Record<[["name", Codec<string>], ["nickName", Codec<string>]], "name" | "nickName", Codec<string>>' is not assignable to type 'Codec<Person>'.
  The types returned by '_d(...)' are incompatible between these types.
    Property 'superPower' is missing in type '{ name: string; } & { nickName: string; }' but required in type 'Person'.

Error Handling

This library intentionally does not check for conditions that would suggest an error. If an error is produced, it is because of invalid input or incompatibility between the input and codec definition. This library prioritizes performance over end-developer DX, as it is not intended for end developers, but rather for tool developers.

Types

Booleans

const bytes = s.bool.encode(true);
const value = s.bool.decode(bytes);

Integers

const bytes = s.u8.encode(9);
const value = s.u8.decode(bytes);

Other such integer types include i8, u16, i16, u32, i32, u64, i64, u128, i128 and compact.

Options

const codec = s.option(s.u8);

const bytes1 = codec.encode(27);
const value1 = codec.decode(bytes1);

const bytes2 = codec.encode(undefined);
const value2 = codec.decode(bytes2);

Arrays

Sized

const codec = s.sizedArray(s.u8, 2);

const bytes = codec.encode([3, 9]);
const value = codec.decode(bytes);

Dynamic

const codec = s.array(s.u8);

const bytes = codec.encode([1, 2, 3, 4, 5]);
const value = codec.decode(bytes);

Tuples

const codec = s.tuple(s.bool, s.u8, s.str);

const bytes = codec.encode([true, 81, "｡＾・ｪ・＾｡"]);
const value = codec.decode(bytes);

Records

const codec = s.record(
  ["name", s.str],
  ["nickName", s.str],
  ["superPower", s.option(s.str)],
);

const bytes = codec.encode({
  name: "Magdalena",
  nickName: "Magz",
  superPower: "Hydrokinesis",
});
const value = codec.decode(bytes);

Unions

Explicitly Discriminated

const codec = s.union(
  (value) => { // Discriminate
    if (typeof value === "string") {
      return 0;
    } else if (typeof value === "number") {
      return 1;
    } else {
      throw new Error("Unreachable");
    }
  },
  s.str, // Member `0`
  s.u8, // Member `1`
);

const bytes1 = codec.encode(27);
const value1 = codec.decode(bytes1);

const bytes2 = codec.encode("Parity");
const value2 = codec.decode(bytes2);

Tagged

const codec = s.taggedUnion(
  "_tag",
  ["dog", ["bark", s.str]],
  ["cat", ["pur", s.str]],
);

const bytes1 = codec.encode({
  _tag: "dog",
  bark: "Roof",
});
const value1 = codec.decode(bytes1);

const bytes2 = codec.encode({
  _tag: "cat",
  pur: "Meow",
});
const value2 = codec.decode(bytes2);

Instance

Sometimes, you may want to instantiate a class with the decoded data / encode data from a class instance. In these situations, we can leverage the instance codec factory.

A common use case for Instance codecs is Error subtypes. Let’s say we want to decode some data into the following Error subtype.

class MyError extends Error {
  constructor(
    readonly code: number,
    readonly message: string,
    readonly payload: {
      a: string;
      b: number;
      c: boolean;
    },
  ) {
    super();
  }
}

We can do so as follows.

const codec = s.instance(
  MyError,
  ["code", s.u8],
  ["message", s.str],
  [
    "payload",
    s.record(
      ["a", s.str],
      ["b", s.u8],
      ["c", s.bool],
    ),
  ],
);

We can now use this codec to encode and decode MyError.

const myError = new MyError(
  1,
  "At war with my Arch system config",
  {
    a: "a",
    b: 2,
    c: true,
  },
);

const encodedBytes = codec.encode(myError);
const decoded = codec.decode(encodedBytes);

Note: executing an equality assertion between myError and decoded will fail, as they contain different stack traces.

Results

Results are initialized with an Ok codec and an Error instance codec.

class MyError {
  constructor(readonly message: string) {}
}
const errorCodec = s.instance(MyError, ["message", s.str]);

const resultCodec = s.Result(errorCodec, s.str);

const errorBytes = resultCodec.encode(new MyError("Uh oh!"));
const errorDecoded = resultCodec.decode(errorBytes);

const okBytes = resultCodec.encode("YES!");
const okDecoded = resultCodec.decode(okBytes);