fix: round money to currency minor units without negative zero (WealthLedger/frontend#118)

src/shared/lib/display.test.ts

@@ -56,6 +56,16 @@ describe("shared display helpers", () => {
 		)).toBe("0,00 ₽");
 	});
 
+	it("rounds to the currency's minor units and drops the sign of a rounded zero", () => {
+		const rounded = { withRoundedCents: true };
+		expect(formatMoney({ currency: "RUB", value: "-0.004" }, rounded)).toBe("0,00 ₽");
+		expect(formatMoney({ currency: "RUB", value: "-0.005" }, rounded)).toBe("-0,01 ₽");
+		expect(formatMoney({ currency: "RUB", value: "999.995" }, rounded)).toBe("1 000,00 ₽");
+		expect(formatMoney({ currency: "JPY", value: "1500.5" }, rounded)).toBe("1 501 JPY");
+		expect(formatMoney({ currency: "KWD", value: "1.2345" }, rounded)).toBe("1,235 KWD");
+		expect(formatMoney({ currency: "ZZZ", value: "1.234" }, rounded)).toBe("1,23 ZZZ");
+	});
+
 	it("builds the brand logo url from an upstream file name", () => {
 		expect(instrumentLogoUrl("sber3.png")).toBe(
 			"https://invest-brands.cdn-tinkoff.ru/sber3x160.png",

src/shared/lib/display.ts

@@ -1,5 +1,5 @@
 const DECIMAL_PART_COUNT = 2;
-const CENTS_PRECISION = 2;
+const DEFAULT_MINOR_UNITS = 2;
 
 type FormatMoneyOptions = {
 	withRoundedCents?: boolean;
@@ -23,6 +23,17 @@ const dateFormatter = new Intl.DateTimeFormat("ru-RU", {
 	year: "numeric",
 });
 
+// ISO 4217 minor units (JPY 0, KWD 3); unknown codes fall back to 2.
+function currencyMinorUnits(currency: string): number {
+	try {
+		return new Intl.NumberFormat("en", { currency, style: "currency" })
+			.resolvedOptions().maximumFractionDigits ?? DEFAULT_MINOR_UNITS;
+	}
+	catch {
+		return DEFAULT_MINOR_UNITS;
+	}
+}
+
 function formatDate(value: string): string {
 	return dateFormatter.format(new Date(value));
 }
@@ -56,8 +67,10 @@ function formatMoney(
 ): string {
 	if (!money) return "—";
 
+	const minorUnits = currencyMinorUnits(money.currency);
+
 	const value = withRoundedCents
-		? roundDecimalToCents(money.value)
+		? roundDecimal(money.value, minorUnits)
 		: money.value;
 
 	const [integerPart = "", fractionPart = ""] = value.split(
@@ -66,7 +79,7 @@ function formatMoney(
 	);
 
 	const normalizedFraction = withRoundedCents || withZeroCents
-		? fractionPart.padEnd(CENTS_PRECISION, "0")
+		? fractionPart.padEnd(minorUnits, "0")
 		: fractionPart;
 
 	const decimal = normalizedFraction
@@ -95,21 +108,25 @@ function isZeroDecimal(value: string) {
 		&& fraction.replaceAll("0", "") === "";
 }
 
-function roundDecimalToCents(value: string): string {
+function roundDecimal(value: string, places: number): string {
 	const [integer = "", fraction = ""] = value.split(".", DECIMAL_PART_COUNT);
-	const sign = integer.startsWith("-") ? "-" : "";
-	const digits = sign ? integer.slice(1) : integer;
-	const roundedFraction = fraction.padEnd(CENTS_PRECISION + 1, "0");
-	const cents = `${digits}${roundedFraction.slice(0, CENTS_PRECISION)}`;
-	const roundingDigit = roundedFraction.at(CENTS_PRECISION) ?? "0";
+	const digits = integer.startsWith("-") ? integer.slice(1) : integer;
+	const paddedFraction = fraction.padEnd(places + 1, "0");
+	const units = `${digits}${paddedFraction.slice(0, places)}`;
+	const roundingDigit = paddedFraction.at(places) ?? "0";
 
-	const rounded = roundingDigit >= "5"
-		? (BigInt(cents) + 1n).toString().padStart(cents.length, "0")
-		: cents;
+	const rounded = (roundingDigit >= "5" ? BigInt(units) + 1n : BigInt(units))
+		.toString()
+		.padStart(places + 1, "0");
 
-	const roundedInteger = rounded.slice(0, -CENTS_PRECISION) || "0";
+	// A value rounded to zero has no sign: "-0.004" → "0.00".
+	const sign = integer.startsWith("-") && /[1-9]/.test(rounded) ? "-" : "";
+	const roundedInteger = rounded.slice(0, rounded.length - places);
+	const roundedFraction = rounded.slice(rounded.length - places);
 
-	return `${sign}${roundedInteger}.${rounded.slice(-CENTS_PRECISION)}`;
+	return roundedFraction
+		? `${sign}${roundedInteger}.${roundedFraction}`
+		: `${sign}${roundedInteger}`;
 }
 
 // Без timeZone Intl берёт часовой пояс браузера пользователя.