ts-variant

No frills polymorphic variants for typescript.

typescript-polymorphic-variants.mp4

Install

npm install @betalyra/ts-variant

Usage

Create a variant using the Variant type constructor:

type Shape = Variant<{
  rectangle: { width: number; height: number };
  circle: { radius: number };
}>;

This will create variants rectangle and circle:

const circle: Shape = { type: "circle", value: { radius: 10 } };
const rectangle: Shape = {
  type: "rectangle",
  value: { height: 10, width: 20 },
};

We can then use a pattern matching library like ts-pattern to do exhaustive pattern matching on the variant:

const show = (shape: Shape) =>
  match(shape)
    .with(
      { type: "circle" },
      ({ value: { radius } }) => `Circle(radius=${radius})`
    )
    .with(
      { type: "rectangle" },
      ({ value: { width, height } }) =>
        `Rectangle(width=${width},height=${height})`
    )
    .exhaustive();
console.log(show(circle)); // Circle(radius=10)
console.log(show(rectangle)); // Rectangle(width=20,height=10)

Note that VS Code will give you type-safe autocomplete for this.

Extending

You can extend this variant with another case by simply using the union type with another variant:

type ExtShape =
  | Shape
  | Variant<{ triangle: { width: number; height: number } }>;