Testing with node:test

test, describe and assert

import { describe, test } from "node:test";
import assert from "node:assert/strict";
import { orderTotal } from "./lib/orders.js";

describe("orderTotal", () => {
  test("adds prices times quantities", () => {
    assert.equal(orderTotal([{ price: 180, qty: 2 }, { price: 60, qty: 1 }]), 420);
  });
  test("an empty order costs 0", () => {
    assert.equal(orderTotal([]), 0);
  });
  test("rejects negative quantities", () => {
    assert.throws(() => orderTotal([{ price: 100, qty: -1 }]));
  });
});

• Run with node --test (it finds *.test.js files). Many teams use Vitest or Jest — same ideas.
• Test HTTP APIs by starting the app and calling it with fetch (as you've been doing!), or with Supertest.
• Fix a bug? First write a test that fails because of it — then it can never quietly come back.