console.assert(new Intl.NumberFormat("es-ES", { notation: "compact" }).format(1_900_000) === "1,9\u00a0M");
console.assert(new Intl.NumberFormat("es-ES", { notation: "compact" }).format(346_000_000) === "346\u00a0M");
console.assert(new Intl.RelativeTimeFormat("es", { numeric: "always" }).format(-4, "month") === "hace 4 meses");
console.assert(new Intl.NumberFormat("ja-JP", { notation: "compact" }).format(1_910_000) === "191万");
console.assert(new Intl.NumberFormat("ja-JP", { notation: "compact", maximumFractionDigits: 1 }).format(340_000_000) === "3.4億");
console.assert(new Intl.RelativeTimeFormat("ja", { numeric: "always" }).format(-4, "month") === "4 か月前");
const d = new Date("2026-10-20T00:00:00Z");
console.assert(new Intl.DateTimeFormat("en-US", { dateStyle: "medium", timeZone: "UTC" }).format(d) === "Oct 20, 2026");
console.assert(new Intl.DateTimeFormat("fr-FR", { dateStyle: "medium", timeZone: "UTC" }).format(d) === "20 oct. 2026");
console.assert(new Intl.DateTimeFormat("he-u-ca-hebrew", { dateStyle: "medium", timeZone: "UTC" }).format(d) === "ט׳ בחשוון תשפ״ז");
console.assert(new Intl.DateTimeFormat("en-u-ca-hebrew", { dateStyle: "medium", timeZone: "UTC" }).format(d) === "9 Heshvan 5787");
console.assert(new Intl.DateTimeFormat("ar-SA-u-ca-islamic-umalqura", { dateStyle: "medium", timeZone: "UTC" }).format(d) === "٩ جمادى الأولى ١٤٤٨ هـ");
console.assert(new Intl.DateTimeFormat("zh-u-ca-chinese", { dateStyle: "long", timeZone: "UTC" }).format(d) === "2026丙午年九月十一");
console.assert(new Intl.DateTimeFormat("ja-u-ca-japanese", { dateStyle: "medium", timeZone: "UTC" }).format(d) === "令和8年10月20日");
const d1 = new Date("2026-10-20T00:00:00Z"), d2 = new Date("2026-10-23T00:00:00Z");
console.assert(new Intl.DateTimeFormat("en-US", { dateStyle: "medium", timeZone: "UTC" }).formatRange(d1, d2) === "Oct 20\u2009–\u200923, 2026");
console.assert(new Intl.DateTimeFormat("fr-FR", { dateStyle: "medium", timeZone: "UTC" }).formatRange(d1, d2) === "20–23 oct. 2026");
console.assert(new Intl.DateTimeFormat("ja-JP", { dateStyle: "medium", timeZone: "UTC" }).formatRange(d1, d2) === "2026/10/20～2026/10/23");
console.assert(new Intl.DisplayNames("en", { type: "language", languageDisplay: "dialect" }).of("fr-CA") === "Canadian French");
console.assert(new Intl.DisplayNames("en", { type: "language", languageDisplay: "standard" }).of("fr-CA") === "French (Canada)");
console.assert(new Intl.DisplayNames("fr", { type: "language", languageDisplay: "dialect" }).of("fr-CA") === "français canadien");
console.assert(new Intl.DisplayNames("zh-Hant", { type: "language" }).of("fr-CA") === "法文（加拿大）");
console.assert([...new Intl.Segmenter("ja", { granularity: "word" }).segment("国際化ライブラリを使う")].map(s => s.segment).join("|") === "国際|化|ライブラリ|を|使う");
console.assert([...new Intl.Segmenter("th", { granularity: "word" }).segment("การทำให้เป็นสากล")].map(s => s.segment).join("|") === "การ|ทำให้|เป็น|สากล");
console.assert(new Intl.Locale("zh-TW").maximize().toString() === "zh-Hant-TW");
console.assert(new Intl.NumberFormat("de-CH", { style: "currency", currency: "CHF" }).format(12345.67) === "CHF\u00a012'345.67");
console.assert(new Intl.NumberFormat("ja-JP", { style: "currency", currency: "JPY", notation: "compact" }).format(100000) === "￥10万");
console.assert(new Intl.NumberFormat("en-US", { style: "unit", unit: "kilometer", unitDisplay: "short" }).format(5) === "5 km");
console.assert(new Intl.NumberFormat("en-US", { style: "unit", unit: "mile", unitDisplay: "short" }).format(3.1) === "3.1 mi");
console.assert(new Intl.DateTimeFormat("ja-u-ca-japanese", { era: "long", year: "numeric", month: "long", day: "numeric", weekday: "long", hour: "2-digit", minute: "2-digit", timeZoneName: "long", timeZone: "Europe/Paris" }).format(new Date("2026-10-20T14:30:00+02:00")) === "令和8年10月20日火曜日 14:30 中央ヨーロッパ夏時間");
console.assert(new Intl.DateTimeFormat("fr-FR", { dateStyle: "medium", timeZone: "UTC" }).formatRange(new Date("2026-10-20T00:00:00Z"), new Date("2026-10-23T00:00:00Z")) === "20–23 oct. 2026");
console.assert(new Intl.RelativeTimeFormat("en").format(3, "day") === "in 3 days");
console.assert(new Intl.RelativeTimeFormat("fr").format(3, "day") === "dans 3 jours");
console.assert(new Intl.Collator("sv").compare("ä", "z") > 0);
console.assert(new Intl.Collator("de").compare("ä", "b") < 0);
console.assert(new Intl.ListFormat("en", { type: "disjunction" }).format(["A", "B", "C"]) === "A, B, or C");
console.assert(new Intl.DisplayNames("en", { type: "region", style: "long" }).of("GB") === "United Kingdom");
console.assert(new Intl.DisplayNames("en", { type: "region", style: "short" }).of("GB") === "UK");
// ICU4X:
// LanguageIdentifierDisplayName::try_new_long_light(locale!("en").into(), langid!("zh-Hans"), Default::default()) => "Simplified Mandarin Chinese"
// WordSegmenter::new_auto(Default::default()).segment_str("ユニコードは、符号化文字集合") => "ユニコード|は|、|符号|化|文字|集合"
s1 = "中ワ況写イノナ開億がゃへ者43俳寺式7沢暮ル材年るあん移酔むぎえす写情主逃69引ぎうぱ何81昇フネイマ本因ぱ不真え断候ら。吉群コクチ賀伝ツエ別写ユリキ施見ら力1取目れづぱぞ然野テ倍展ぴるてけ京答週ネコ入整料勉らげ。";
s2 = "中|ワ|況|写|イノナ|開|億|が|ゃ|へ|者|43|俳|寺|式|7|沢|暮|ル|材|年|る|あん|移|酔|むぎ|え|す|写|情|主|逃|69|引|ぎ|う|ぱ|何|81|昇|フネイマ|本|因|ぱ|不|真|え|断|候|ら|。|吉|群|コクチ|賀|伝|ツエ|別|写|ユリキ|施|見|ら|力|1|取|目|れ|づ|ぱ|ぞ|然|野|テ|倍|展|ぴ|る|て|け|京|答|週|ネコ|入|整|料|勉|ら|げ|。";
console.assert([...new Intl.Segmenter("ja", { granularity: "word" }).segment(s1)].map((v) => v.segment).join("|") === s2);
Curved milestone callout arrow from 2020 to 2026
Chart 1: DateTimeFormatter Performance (sec, lower is better)
Chart 2: DateTimeFormatter Memory Use (log scale bytes, lower is better)
Chart 3: Basic Number Format: Code Size (kB, lower is better)
console.assert(new Intl.Locale("DE-CH").toString() === "de-CH");
console.assert(new Intl.Locale("ES-419").toString() === "es-419");
console.assert(new Intl.DateTimeFormat("ja-u-ca-japanese", { dateStyle: "medium", timeStyle: "short", timeZone: "UTC" }).format(new Date("2026-10-20T14:30:00Z")) === "令和8年10月20日 14:30");
// Rust solution:
use icu::calendar::Date;
use icu::datetime::{fieldsets::YMD, DateTimeFormatter};
use icu::locale::Locale;
use icu::time::{DateTime, Time};
let locale: Locale = "ja-u-ca-japanese".parse()?;
let dtf = DateTimeFormatter::try_new(
locale.into(),
YMD::medium().with_time_hm(), // YMDT
)?;
let dt = DateTime {
date: Date::try_new_iso(2026, 10, 20)?,
time: Time::try_new(14, 30, 0, 0)?,
};
println!("{}", dtf.format(&dt));
// => 令和8年10月20日 14:30
ICU4X compiled_data root fallback:
DateTimeFormatter::try_new(locale!("ccp").into(), YMD::medium()).unwrap().format(&Date::try_new_iso(2026, 10, 20).unwrap()) // => "2026 M10 20"
https://github.com/unicode-org/icu4x/blob/main/tutorials/data-provider-runtime.md
console.assert(new Intl.DateTimeFormat("ccp", { dateStyle: "medium", timeZone: "UTC" }).format(new Date("2026-10-20T00:00:00Z")) === "𑄸𑄶 𑄃𑄧𑄇𑄴𑄑𑄮𑄝𑄧𑄢𑄴, 𑄸𑄶𑄸𑄼");
console.assert(["en-GB", "az", "zh-Hans"].map(c => new Intl.DisplayNames("en", { type: "language", languageDisplay: "dialect", style: "long" }).of(c)).join(" | ") === "British English | Azerbaijani | Simplified Chinese");
console.assert(["en-GB", "az", "zh-Hans"].map(c => new Intl.DisplayNames("en", { type: "language", languageDisplay: "dialect", style: "short" }).of(c)).join(" | ") === "UK English | Azeri | Simplified Chinese");
console.assert(["en-GB", "az", "zh-Hans"].map(c => new Intl.DisplayNames("en", { type: "language", languageDisplay: "standard", style: "long" }).of(c)).join(" | ") === "English (United Kingdom) | Azerbaijani | Chinese (Simplified)");
// ICU4X Long, Menu & TryWriteable fallback tests:
// https://github.com/unicode-org/icu4x/blob/main/components/locale/src/names/language.rs#L57-L181
// https://github.com/unicode-org/icu4x/blob/main/components/locale/src/names/language.rs#L2080-L2117
console.assert(["fr-CA", "zh-Hans", "hi-Deva-IN"].map(c => new Intl.DisplayNames("fr", { type: "language", languageDisplay: "dialect" }).of(c)).join(" | ") === "français canadien | chinois simplifié | hindi (dévanagari, Inde)");
// ICU4X Tiny / Light / Heavy tiers (locale!("fr") with [langid!("fr-CA"), langid!("zh-Hans"), langid!("hi-Deva-IN")]):
// - try_new_tiny: [Ok("français (Canada)"), Err("zh (Hans)"), Err("hi (Deva, IN)")]
// - try_new_light: [Ok("français canadien"), Ok("chinois simplifié"), Err("hindi (Deva, Inde)")]
// - try_new_heavy: [Ok("français canadien"), Ok("chinois simplifié"), Ok("hindi (dévanagari, Inde)")]
// https://github.com/unicode-org/icu4x/blob/main/components/locale/src/names/language.rs#L429-L460
console.assert(new Intl.DateTimeFormat("en-US", { dateStyle: "medium", timeZone: "UTC" }).formatRange(new Date("2026-10-20T00:00:00Z"), new Date("2026-10-23T00:00:00Z")) === "Oct 20\u2009–\u200923, 2026");
console.assert(new Intl.DateTimeFormat("en-US", { dayPeriod: "long", timeZone: "UTC" }).format(new Date("2026-10-20T09:00:00Z")) === "in the morning");
console.assert(new Intl.DateTimeFormat("en-US", { dayPeriod: "long", timeZone: "UTC" }).format(new Date("2026-10-20T23:00:00Z")) === "at night");
console.assert(new Intl.NumberFormat("en-US", { style: "currency", currency: "USD" }).format(12345.67) === "$12,345.67");
console.assert(new Intl.NumberFormat("en-US", { style: "currency", currency: "USD", currencyDisplay: "code" }).format(12345.67) === "USD\u00a012,345.67");
console.assert(new Intl.NumberFormat("en-US", { style: "currency", currency: "USD", notation: "compact" }).format(12345.67) === "$12K");
console.assert(new Intl.NumberFormat("en-US", { style: "currency", currency: "USD", currencySign: "accounting" }).format(-12345.67) === "($12,345.67)");
console.assert(new Intl.NumberFormat("de-CH", { style: "currency", currency: "CHF" }).format(-12345.67) === "CHF-12'345.67");
// ICU4X currency tests (including try_new_no_currency):
// https://github.com/unicode-org/icu4x/blob/main/components/experimental/src/dimension/currency/format.rs