Review brief

Beginner20 min

A marketplace is replacing hand-built price labels with `Intl.NumberFormat`. Its pricing API returns signed integer minor-unit amounts, while the caller supplies the shopper's resolved locale and one of the marketplace's supported currencies.

Behavioral contract

amountMinor is a finite safe integer with absolute value at most 1,000,000,000,000, expressed in the currency's minor unit, and may be negative for refunds or credits.
The supported currencies are USD with two fractional digits, JPY with zero, and KWD with three.
Format the mathematically equivalent major-unit amount with `Intl.NumberFormat` using the exact supplied locale, currency style, and currency code.
Do not override the formatter's locale-specific grouping, decimal, sign, currency-symbol placement, or default fraction digits.
The caller supplies a valid supported locale and currency, so this helper need not implement fallback selection.

Review instructions

1Review only the changed code against the behavioral contract in the brief.
2Anchor each finding to the narrowest useful changed line and give a concrete display consequence.
3Recommend a direction rather than writing replacement code.
4Prioritize no more than four findings.

Learning objectives

• Keep minor-unit conversion aligned with currency exponents
• Trace locale input through an internationalization API
• Check signed money values and representative tests

src/currency-display.tsTypeScript

@@ -0,0 +1,14 @@

1+export type SupportedCurrency = "USD" | "JPY" | "KWD";

3+export function formatCurrency(

4+ amountMinor: number,

5+ currency: SupportedCurrency,

6+ locale: string,

7+): string {

8+ const majorAmount = Math.abs(amountMinor) / 100;

10+ return new Intl.NumberFormat("en-US", {

11+ style: "currency",

12+ currency,

13+ }).format(majorAmount);

14+}

Reference solution

The findings below are ordered by priority.

Findings

1. Every currency is converted as though it had two minor digits

Location: src/currency-display.ts:8

Dividing every amount by 100 is valid for USD only. A JPY value of 1,234 is displayed as roughly 12 yen instead of 1,234 yen, while 1,234 KWD minor units become about 12.34 dinars instead of 1.234. These are material price changes, not merely presentation differences.

Map each supported currency to its documented exponent and divide by 10 raised to that exponent before handing the major-unit value to Intl.NumberFormat.

2. The shopper's locale is ignored

Location: src/currency-display.ts:10

The formatter always uses en-US even though locale is a required input. Shoppers in other locales receive US grouping, decimal separators, sign conventions, and symbol placement rather than the requested localized representation.

Pass the supplied locale to Intl.NumberFormat and let the platform apply its locale data.

3. Refunds and credits lose their negative sign

Location: src/currency-display.ts:8

Math.abs turns every negative minor-unit amount positive. A refund such as -1,234 USD is displayed as a $12.34 charge-like value, reversing the financial meaning shown to the user.

Preserve the sign during minor-to-major conversion and allow the internationalization formatter to render it.

4. The test exercises only the one combination that fits the assumptions

Location: src/currency-display.test.ts:6-8

Positive USD in en-US has two minor digits and matches the hard-coded locale, so the test passes despite all three production defects. It gives no coverage to alternate exponents, locale punctuation, or signed values.

Add representative JPY and KWD cases, at least one non-US locale, and a negative amount. Derive expected strings from the stated locale behavior rather than weakening assertions to substring checks.

Reasonable non-findings

Using Intl.NumberFormat with style: "currency" is appropriate.
The currency code is passed through to the formatter correctly.
The helper may rely on the caller guarantees for safe integers and supported locale identifiers.