# Currency formatter
Format a decimal amount for `en-US`, with currency selected by its ISO 4217 numeric code. Formatting runs locally in your browser with the same WebAssembly component used by the CLI.
The component accepts one ASCII decimal matching `-?[0-9]+(\.[0-9]+)?`. It groups thousands, uses the selected currency's ISO minor-unit count, and rounds halfway values away from zero. It reads decimal text rather than an IEEE-754 number, so inputs such as `0.005` are not changed by binary floating-point conversion before formatting. Invalid syntax and unsupported codes trap instead of producing a plausible amount.
## Use the component
```bash
printf %s '1234567.895' | qip run modules/utf8/currency-format-en-us.wasm '?currency=392'
# ¥1,234,568
```
`currency-format-en-us.wasm` keeps the locale and its formatting data fixed while the integer uniform selects one of 155 current country currencies plus XDR. Metals, testing, no-currency, fund, accounting-unit, and indexed-unit codes remain outside its scope. The 2.2 KiB component has no dependency on the browser, operating system locale, or host ICU version. The earlier `currency-format-usd-en-us.wasm` remains as a fixed-USD compatibility component.
Its executable specification is `compliance/currency-format-en-us.comply.wasm`. Each supported currency is dueled against JavaScript `Intl.NumberFormat`, including zero-digit JPY and KRW, three-digit BHD, non-breaking spacing for alphabetic symbols, negative zero, rounding carries, and generated decimal inputs.
## Convert alphabetic codes
The companion component converts all 178 current ISO 4217 alphabetic codes to three-digit ASCII numeric codes. It keeps leading zeroes, which are significant in the standard representation:
```bash
printf %s AUD | qip run modules/utf8/iso-4217-alpha-to-numeric.wasm
# 036
printf %s BHD | qip run modules/utf8/iso-4217-alpha-to-numeric.wasm
# 048
```
Input is exactly three uppercase ASCII letters. Unknown, lowercase, or malformed codes trap. Its compliance component exhaustively declares all 178 mappings from the SIX Group ISO 4217 List One published 2026-01-01, plus rejection cases.
## Where this can go next
The next variants should force the design to handle real differences rather than add more names for the same rules:
- **Cash rounding:** the formatter uses standard fraction digits. Currency-specific cash increments, such as rounding to the nearest five cents, remain a separate policy.
- **USD in more locales:** `fr-FR` changes separators, symbol placement, and spacing; `hi-IN` adds lakh grouping; `ar` adds native digits and bidirectional text concerns.
- **Pinned CLDR generation:** once those dimensions are understood, a generator can read a committed CLDR release and emit the tables, components, and compliance fixtures. The CLDR revision should then become part of the artifact identity so an upgrade is reviewable.
- **A broader number formatter:** decimal and percent styles can reuse parsing, grouping, digits, and rounding. Compact notation and arbitrary option bags should wait until the smaller components show which shared runtime is actually useful.
This component is intentionally narrower than `Intl.NumberFormat`. It does not accept exponent notation, `NaN`, infinity, signs other than a leading minus, configurable fraction digits, or runtime locale selection. Use host `Intl` when ambient platform behavior is acceptable and you need its full option surface.