BillTracker/tests/moneyProperties.test.js

'use strict';

// Property-based proof of the money invariants. A financial app must not sample
// its cent math — it must hold for all inputs. Fuzzes utils/money.mts.
const test = require('node:test');
const assert = require('node:assert/strict');
const fc = require('fast-check');

let money;
test.before(async () => {
  money = await import('../utils/money.mts'); // ESM .mts
});

// Realistic dollar amounts for this app (up to ±$100M), finite only.
const dollars = () => fc.double({ min: -1e8, max: 1e8, noNaN: true });

test('toCents always yields an integer for finite input', () => {
  fc.assert(fc.property(dollars(), (d) => Number.isInteger(money.toCents(d))));
});

test('dollars → cents → dollars equals roundMoney (no float drift)', () => {
  fc.assert(
    fc.property(dollars(), (d) => money.fromCents(money.toCents(d)) === money.roundMoney(d)),
  );
});

test('sumMoney is cent-exact (equals the sum taken in integer cents)', () => {
  fc.assert(
    fc.property(fc.array(dollars(), { maxLength: 60 }), (arr) => {
      const cents = arr.reduce((s, d) => s + money.toCents(d), 0);
      return money.sumMoney(arr) === money.fromCents(cents);
    }),
  );
});

test('integer cents round-trip back to themselves', () => {
  fc.assert(
    fc.property(
      fc.integer({ min: -1e9, max: 1e9 }),
      (c) => money.toCents(money.fromCents(c)) === c,
    ),
  );
});

test('classic float-drift cases are exact', () => {
  assert.equal(money.sumMoney([0.1, 0.2]), 0.3);
  assert.equal(money.sumMoney([0.1, 0.1, 0.1]), 0.3);
  assert.equal(money.toCents(1.005), 101); // half-up on the shortest round-tripping decimal
  assert.equal(money.toCents('$1,234.56'), 123456);
});

test('invalid inputs never produce a bogus number', () => {
  assert.equal(money.toCents(null), null);
  assert.equal(money.toCents(''), null);
  assert.equal(money.fromCents(null), null);
  assert.equal(money.fromCents(undefined), null);
  assert.ok(Number.isNaN(money.toCents('not-money')));
});