Amounts and currencies

const DECIMALS = 2;
const AMOUNT_PATTERN = /^-?\d+(\.\d+)?$/;

// "250000.00", 250000 or 1200.5 -> integer minor units (25000000n, 120050n)
export function toMinorUnits(value) {
  const text = String(value).trim();
  if (!AMOUNT_PATTERN.test(text)) throw new Error(`Not a plain decimal amount: ${text}`);
  const negative = text.startsWith('-');
  const [whole, fraction = ''] = text.replace('-', '').split('.');
  if (fraction.replace(/0+$/, '').length > DECIMALS) {
    throw new Error(`More than ${DECIMALS} decimal places: ${text}`); // don't round money silently
  }
  const units = BigInt(whole) * 10n ** BigInt(DECIMALS) + BigInt(fraction.padEnd(DECIMALS, '0').slice(0, DECIMALS));
  return negative ? -units : units;
}

// Integer minor units -> the JSON number to send (120050n -> 1200.5)
export function toRequestAmount(minorUnits) {
  return Number(minorUnits) / 10 ** DECIMALS;
}

toMinorUnits(250000);      // 25000000n
toMinorUnits('250000.00'); // 25000000n
toMinorUnits('1500.5');    // 150050n
toRequestAmount(120050n);  // 1200.5

from decimal import Decimal, InvalidOperation

CENT = Decimal("0.01")


def to_decimal(value):
    # str() first, so 0.1 arriving as a JSON float doesn't become 0.1000000000000000055...
    try:
        amount = Decimal(str(value))
    except InvalidOperation:
        raise ValueError(f"Not an amount: {value!r}")
    if amount != amount.quantize(CENT):
        raise ValueError(f"More than 2 decimal places: {value!r}")  # don't round money silently
    return amount.quantize(CENT)


def to_request_amount(amount):
    # Decimal -> the JSON number to send; json.dumps can't write a Decimal
    return float(amount.quantize(CENT))


to_decimal(250000)        # Decimal('250000.00')
to_decimal("250000.00")   # Decimal('250000.00')
to_decimal("1500.5")      # Decimal('1500.50')
to_request_amount(Decimal("1200.50"))  # 1200.5