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Money, the type

28 September 2026·2 min read

Exact decimals were the start. An amount also has a currency, minor units and rounding rules, and a split that must not lose a cent.

Exact decimals fix 0.1 + 0.2. They do not stop a program from adding dollars to euros, keeping 12.345 dollars, or splitting 100 dollars three ways into 99.99. Money needs more than a number.

An amount and its currency

let price = money.of(19.99m, "USD")             // { amount: 19.99m, currency: "USD" }money.of(form.amount, "EUR") ?? reply 400       // text from a form; an error value if badmoney.of("12.345", "USD")                       // err: more decimals than USD hasmoney.add(a, b)                                  // an error if a and b differ in currencymoney.format(price, "de-DE")                    // "19,99 $"

An amount always has its currency's minor units (ISO 4217: two for USD, none for JPY, three for BHD). It is a plain map, so it goes into JSON, the database and the browser as it is, and money checks it each time it gets one.

Two currencies are never a sum

add, sub and compare take amounts of one currency. Mixing two is an error, never a silent total. Converting is something you do on purpose, with a rate you chose.

Splitting without losing a cent

money.allocate(money.of(100, "USD"), [1, 1, 1])  // 33.34, 33.33, 33.33

allocate splits by ratios and gives the leftover minor units, one each, to the first shares, so the parts always add up to the whole. Refunds per night, a bill split three ways, a fee shared between sellers.

The same on both sides

money works in components too, with the same rules, so the total in the cart is the total on the invoice, to the cent.

Read more: Money in the docs.