Never Store Money as a Float

// ❌ NEVER do this
const balance = 100.50;
balance + 0.05;        // 100.55000000000001 (floating point error)
balance * 1000;        // 100499.99999999999 (more drift)

// ✓ ALWAYS use integer minor units + currency code
type Money = { amount: bigint; currency: "USD" | "EUR" | "INR" | "JPY" };

const balance: Money = { amount: 10050n, currency: "USD" }; // $100.50

function add(a: Money, b: Money): Money {
  if (a.currency !== b.currency) throw new Error("currency mismatch");
  return { amount: a.amount + b.amount, currency: a.currency };
}

function fmt(m: Money): string {
  const decimals = CURRENCY_DECIMALS[m.currency]; // USD=2, JPY=0, BHD=3
  const div = 10n ** BigInt(decimals);
  const whole = m.amount / div;
  const fraction = m.amount % div;
  return `${whole}.${fraction.toString().padStart(decimals, "0")} ${m.currency}`;
}

// ISO 4217 currency decimals (NOT always 2):
//   USD, EUR, INR, GBP: 2
//   JPY, KRW:          0  (one yen / one won is the smallest unit)
//   BHD (Bahrain):     3  (1 dinar = 1000 fils)
//   CLF (Chilean UF):  4

// Persistence:
//   Postgres: NUMERIC(20, 0) for amount (integer; no decimal needed)
//             VARCHAR(3) for currency code
//   MongoDB:  amount as Long; currency as enum string
//   Never:    DOUBLE PRECISION or FLOAT for money

// Display:
//   Format AT THE EDGE: convert minor units to display string at API/UI boundary
//   Never round inside the system; only at the display layer