ts-pattern exhaustive matching on discriminated unions: working versions (tested 2026-10-15)

{
  "name": "ts-pattern-discriminated-unions",
  "private": true,
  "type": "module",
  "scripts": {
    "test": "node --import tsx --test test/*.test.ts"
  },
  "dependencies": {
    "ts-pattern": "5.9.2"
  },
  "devDependencies": {
    "@types/node": "26.6.5",
    "tsx": "4.23.15"
  }
}

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

export type Event =
  | { type: "user_created"; id: string; email: string }
  | { type: "user_deleted"; id: string; reason: string }
  | { type: "payment"; id: string; amount: number; currency: "USD" | "EUR" }
  | { type: "refund"; id: string; amount: number; currency: "USD" | "EUR"; paymentId: string };

export function describeEvent(e: Event): string {
  return match(e)
    .with({ type: "user_created" }, (x) => `created ${x.email}`)
    .with({ type: "user_deleted", reason: P.string.minLength(1) }, (x) => `deleted ${x.id}: ${x.reason}`)
    .with({ type: "payment", currency: "USD", amount: P.number.gt(0) }, (x) => `charge $${x.amount}`)
    .with({ type: "payment", currency: "EUR" }, (x) => `charge €${x.amount}`)
    .with({ type: "refund" }, (x) => `refund ${x.amount} ${x.currency} for ${x.paymentId}`)
    .exhaustive();
}

export function totalUsd(events: Event[]): number {
  return events.reduce((sum, e) => sum + match(e)
    .with({ type: "payment", currency: "USD" }, (x) => x.amount)
    .with({ type: "refund", currency: "USD" }, (x) => -x.amount)
    .otherwise(() => 0), 0);
}

import { test } from "node:test";
import assert from "node:assert/strict";
import { describeEvent, totalUsd, type Event } from "../src/events.ts";

test("exhaustive descriptions", () => {
  assert.equal(describeEvent({ type: "user_created", id: "1", email: "[email protected]" }), "created [email protected]");
  assert.equal(describeEvent({ type: "payment", id: "p1", amount: 12, currency: "USD" }), "charge $12");
  assert.equal(describeEvent({ type: "refund", id: "r1", amount: 5, currency: "USD", paymentId: "p1" }), "refund 5 USD for p1");
});

test("totals usd payments and refunds", () => {
  const events: Event[] = [
    { type: "payment", id: "1", amount: 20, currency: "USD" },
    { type: "payment", id: "2", amount: 5, currency: "EUR" },
    { type: "refund", id: "3", amount: 3, currency: "USD", paymentId: "1" },
  ];
  assert.equal(totalUsd(events), 17);
});