What is the difference between Intl.NumberFormat styles?

The style option selects what is formatted: 'decimal' for plain numbers, 'currency' (requires a currency code) for money, 'percent' which multiplies by 100 and adds a percent sign, and 'unit' (requires a unit) for measurements like kilometers. Each respects the locale's grouping and decimal separators.

Why does Intl.PluralRules return 'one' for some non-1 numbers?

Plural categories are language-specific, not just singular vs plural. English uses only 'one' and 'other', but Arabic has six categories (zero, one, two, few, many, other) and Polish distinguishes 'few' and 'many'. PluralRules returns the CLDR category so you can pick the correct message form.

How do I format relative times like '3 days ago'?

Use Intl.RelativeTimeFormat: new Intl.RelativeTimeFormat('en', { numeric: 'auto' }).format(-3, 'day') gives '3 days ago', and format(1, 'day') gives 'tomorrow'. The numeric:'auto' option enables idiomatic words like yesterday/tomorrow instead of always saying '1 day ago'.

What does Intl.Collator do that String.localeCompare does not?

Intl.Collator is the configurable engine behind localeCompare. Creating one Collator and reusing its compare function is faster for sorting large arrays, and it exposes options like sensitivity (base/accent/case), numeric (so 'a2' sorts before 'a10') and caseFirst that you cannot set as conveniently otherwise.

Are Intl constructors expensive to create?

Yes, relatively — building a formatter loads locale data and is far slower than calling .format(). Create the formatter once and reuse it across many values rather than constructing a new one inside a loop. The output of this tool is computed in your browser using the same constructors.

JavaScript Intl API Reference

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The Intl namespace provides locale-aware formatting and comparison built into JavaScript. This reference lists the main constructors and runs them live in your browser so you can see real output.

How it works

Each Intl constructor takes a locale (a BCP 47 tag like en-US, de-DE, ar-EG) and an options object, then exposes a .format() (or .compare(), .select()) method:

new Intl.NumberFormat("de-DE", { style: "currency", currency: "EUR" }).format(1234.5);
// → "1.234,50 €"

new Intl.RelativeTimeFormat("en", { numeric: "auto" }).format(-1, "day");
// → "yesterday"

new Intl.PluralRules("ar-EG").select(3);
// → "few"

Locale data ships with the engine, so the same code adapts grouping separators, decimal marks, currency placement, plural categories and collation order per language automatically.

Tips and notes

Reuse formatters. Constructing an Intl object is expensive (it loads locale data); calling .format() is cheap. Build once, format many.
Plural categories are language-specific. Don’t assume one/other — Arabic has six. Use PluralRules.select(n) to choose the right message form.
numeric: "auto" on RelativeTimeFormat produces idiomatic words (yesterday, next week).
Intl.Collator with numeric: true sorts a2 before a10 (natural sort).

The live demo below uses these exact constructors on your machine, so its output reflects your runtime’s locale support.