Pattern Matching in TypeScript

pattern-matching.ts

//
// This is a code example from the blog post:
//
// https://andrekoenig.de/articles/pattern-matching-in-typescript
//

import { P, match } from "ts-pattern";

type ProductWasAddedToCartEvent = Readonly<{
  type: "ProductWasAddedToCart";
  payload: Readonly<{
    productId: string;
    quantity: number;
  }>;
}>;

type ProductWasRemovedFromCart = Readonly<{
  type: "ProductWasRemovedFromCart";
  payload: Readonly<{
    productId: string;
  }>;
}>;

type CartEvent = ProductWasAddedToCartEvent | ProductWasRemovedFromCart;

const events: CartEvent[] = [
  {
    type: "ProductWasAddedToCart",
    payload: {
      productId: "1",
      quantity: 2,
    },
  },
  {
    type: "ProductWasRemovedFromCart",
    payload: {
      productId: "1",
    },
  },
  {
    type: "ProductWasAddedToCart",
    payload: {
      productId: "3",
      quantity: 300,
    },
  },
  {
    type: "ProductWasAddedToCart",
    payload: {
      productId: "4",
      quantity: 320,
    },
  },
];

function ProductsWithHighQuantityWereAddedToCart(event: CartEvent) {
  return match(event)
    .with(
      {
        type: "ProductWasAddedToCart",
        payload: { quantity: P.number.gt(100) },
      },
      (event) => event,
    )
    .otherwise(() => undefined);
}

const result = events.filter(ProductsWithHighQuantityWereAddedToCart);

console.log(result);