An amount field that stores integer minor units and adapts its precision to the selected currency, instead of assuming two decimal places.
How much of a money input's correctness can be pushed into the interface layer, before the API is involved at all?
US Dollar uses 2 decimals. Formatting is applied on blur so it never fights what you are typing.
Two failure modes that are cheap to prevent in the input and expensive to fix downstream.
Floating point. 0.1 + 0.2 is 0.30000000000000004. Money held as a
JavaScript number with a decimal point is money that will eventually be wrong
by a cent, in a system where being wrong by a cent is the whole problem. The
input therefore never holds a decimal value: it holds an integer count of minor
units, and the decimal point exists only in the rendering.
Assumed precision. Two decimal places is a Western-currency habit, not a rule. JPY has no minor unit; KWD, BHD and OMR have three. An input hard-coded to two either refuses valid amounts or silently truncates them.
/** Digits typed by the user → integer minor units. No floating point involved. */
function toMinorUnits(input: string, decimals: number): number | null {
const cleaned = input.replace(/[^\d.]/g, "");
if (cleaned === "" || cleaned === ".") return null;
const [whole = "0", fraction = ""] = cleaned.split(".");
const padded = fraction.slice(0, decimals).padEnd(decimals, "0");
const combined = `${whole}${padded}`.replace(/^0+(?=\d)/, "");
const parsed = Number.parseInt(combined || "0", 10);
return Number.isSafeInteger(parsed) ? parsed : null;
}Switching currency re-quantises the amount to the new currency's precision rather
than keeping digits that currency cannot represent. Entering 1250.75 and
switching to JPY gives 1251, not a value with a hidden fractional part waiting
to surprise someone at settlement.
inputMode="decimal" gives a numeric keypad on mobile while keeping a text
input, so formatting characters remain allowed.aria-describedby,
rather than being discoverable only by trying it.Negative amounts are not handled. In most business-finance contexts, direction
belongs to the transaction type, not to the sign of the amount, but that is a
product decision rather than a settled one. Paste handling also needs work:
pasting $1,250.75 currently works by accident, not by design.