Intl API Locale Codes — JavaScript Internationalization

The JavaScript Intl API

The Intl object is the namespace for the ECMAScript Internationalization API. It provides language-sensitive string comparison, number formatting, date formatting, and more — all driven by BCP 47 locale identifiers.

Core Intl Constructors

Constructor	Purpose	Example
Intl.DateTimeFormat	Format dates/times	new Intl.DateTimeFormat("ja-JP")
Intl.NumberFormat	Format numbers/currencies	new Intl.NumberFormat("de-DE")
Intl.DisplayNames	Localized names	new Intl.DisplayNames(["en"])
Intl.RelativeTimeFormat	Relative time	"3 days ago"
Intl.ListFormat	List formatting	"A, B, and C"
Intl.Collator	String comparison	locale-aware sorting
Intl.Segmenter	Text segmentation	word/sentence boundaries

Locale Resolution

The Intl API performs locale negotiation to find the best available locale:

// Request Japanese, fall back to English
const fmt = new Intl.DateTimeFormat(["ja-JP", "en-US"]);
console.log(fmt.resolvedOptions().locale); // "ja-JP" (if available)

Practical Examples

Date formatting by locale:

const date = new Date("2026-02-28");
new Intl.DateTimeFormat("en-US").format(date); // "2/28/2026"
new Intl.DateTimeFormat("ja-JP").format(date); // "2026/2/28"
new Intl.DateTimeFormat("de-DE").format(date); // "28.2.2026"

Currency formatting:

const opts = { style: "currency", currency: "USD" };
new Intl.NumberFormat("en-US", opts).format(1234.5); // "$1,234.50"
new Intl.NumberFormat("ja-JP", opts).format(1234.5); // "$1,234.50"
new Intl.NumberFormat("de-DE", opts).format(1234.5); // "1.234,50 $"

Display names:

const dn = new Intl.DisplayNames(["en"], { type: "language" });
dn.of("ja"); // "Japanese"
dn.of("zh-Hans"); // "Simplified Chinese"
dn.of("sr-Latn"); // "Serbian (Latin)"

Browser Support

All Intl constructors are supported in every modern browser (Chrome 24+, Firefox 29+, Safari 10+, Edge 12+). Intl.Segmenter is the newest addition (Chrome 87+, Safari 15.4+).