json-tag

TypeScript support for JSON#.

Utility types and runtime helpers for the JSON# convention.

Install

npm install @immersus-machina/json-tag

Usage

Define your types:

import type { JsonTagged } from "@immersus-machina/json-tag";

type Circle = JsonTagged<"Circle", { radius: number }>;
type Rectangle = JsonTagged<"Rectangle", { width: number; height: number }>;
type Shape = Circle | Rectangle;

Narrow with type safety:

import { hasJsonTag } from "@immersus-machina/json-tag";

const shape = JSON.parse(json) as Shape;

if (hasJsonTag(shape, "Circle")) {
  shape.radius; // compiler knows this exists
}

Create tagged objects:

import { toJsonTag } from "@immersus-machina/json-tag";

const circle = toJsonTag("Circle", { radius: 4 });
// { "#type": "Circle", radius: 4 }

API

| Export | Kind | Description | |---|---|---| | JsonTagged<Tag, Fields> | type | Tagged object type | | JsonTagOf<T> | type | Extract tag from type | | JsonTagVariant<Union, Tag> | type | Narrow union to variant | | hasJsonTag(value, tag) | function | Type predicate — narrows to variant | | isJsonTagged(value) | function | Type guard — checks for #type string | | jsonTagOf(value) | function | Extract #type value or undefined | | toJsonTag(tag, fields) | function | Create tagged object | | TYPE_FIELD | const | "#type" |

Zero dependencies

No runtime dependencies.