Internationalization with Intl

This chapter follows Dates and Temporal.

The Intl namespace (ECMA-402) provides locale-aware formatting and collation. Engines ship CLDR-based data; you pass BCP 47 locale tags ("de-DE", "en-US", "ja-JP").

Intl.DateTimeFormat

Format instants or Date values for display—time zone and calendar options are explicit.

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const fmt = new Intl.DateTimeFormat("de-DE", {
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  dateStyle: "full",
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  timeStyle: "short",
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  timeZone: "Europe/Berlin",
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});
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fmt.format(new Date("2026-11-01T12:00:00Z"));

Use with Dates and Temporal once you have a definite instant or zoned datetime.

Intl.NumberFormat

Currency, percent, unit, and compact notation:

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new Intl.NumberFormat("en-US", {
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  style: "currency",
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  currency: "USD",
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}).format(1234.5); // "$1,234.50"
5

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new Intl.NumberFormat("de-DE", {
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  style: "unit",
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  unit: "kilometer",
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}).format(42);

Intl.RelativeTimeFormat

“3 days ago” / “in 2 hours” with plural rules:

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const rtf = new Intl.RelativeTimeFormat("en", { numeric: "auto" });
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rtf.format(-1, "day"); // "yesterday"
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rtf.format(2, "hour"); // "in 2 hours"

Intl.Collator

Locale-aware string sort (not plain localeCompare scattered everywhere):

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const collator = new Intl.Collator("sv");
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["z", "ä", "a"].sort(collator.compare);

Intl.ListFormat

Natural-language joining of arrays:

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new Intl.ListFormat("en", { type: "conjunction" }).format([
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  "HTML",
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  "CSS",
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  "JavaScript",
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]);
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// "HTML, CSS, and JavaScript"

Intl.PluralRules and messages

Choose grammatical plural for a count:

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const pr = new Intl.PluralRules("en");
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pr.select(1); // "one"
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pr.select(2); // "other"

Pair with your message catalog (ICU MessageFormat, i18next, etc.)—Intl does not translate strings by itself.

Locale resolution

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Intl.DateTimeFormat().resolvedOptions();
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// { locale, calendar, numberingSystem, timeZone, ... }

Pass one locale string, or an array for fallback: ["de-CH", "de", "en"].

Limits

• Formatting only—translation and copy live in your resource files.
• Data updates ship with the engine; exotic locales may need polyfills on older runtimes.
• Intl does not parse arbitrary user date strings; parse with Temporal/Date first, then format.

This is the last chapter in Standard Library. For collection types, start with Arrays in JavaScript.