How do you format dates, numbers, and plurals per locale with Intl?

Intl is the built-in internationalization API (ECMA-402). Each constructor takes (locales, options) and returns a formatter with format() (and often formatToParts()); toLocaleString() and friends are shortcuts that build one internally.

What interviewers look for:

• Do not hand-roll patterns. Field order, separators, digits, currency placement and list grammar all vary by locale, and Intl gets them from CLDR data.
• The time zone is separate from the locale. DateTimeFormat uses the host's zone unless you pass an IANA timeZone. Formatting server-side without one gives different strings on different machines.
• Plurals are categories, not n === 1. English has one/other; Polish has one/few/many/other; Arabic has six. Select a category and map it to a message.
• dateStyle/timeStyle cannot be mixed with field options such as hour. V8 throws a TypeError at construction. hour12 overrides hourCycle.
• Reuse formatters. Construction loads locale data. Build once at module level or memoize per locale and options.
• Output is data-dependent. The same call can differ across engines and ICU/CLDR versions, so never parse formatted strings or assert on them in cross-environment tests.

// Dates: the zone is explicit, the locale decides order and wording
const when = new Date('2026-03-08T17:30:00Z');
const opts = { timeZone: 'Asia/Kolkata', dateStyle: 'long', timeStyle: 'short' };
console.log(new Intl.DateTimeFormat('en-GB', opts).format(when));
// → 8 March 2026 at 23:00
console.log(new Intl.DateTimeFormat('de-DE', opts).format(when));
// → 8. März 2026 um 23:00

// Numbers: currency, compact, unit
const usd = new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' });
console.log(usd.format(1234.5), new Intl.NumberFormat('de-DE', { style: 'currency', currency: 'EUR' }).format(1234.5));
// → $1,234.50 1.234,50 €
console.log(new Intl.NumberFormat('en-US', { notation: 'compact' }).format(1_234_567));
// → 1.2M
console.log(new Intl.NumberFormat('en-US', { style: 'unit', unit: 'kilometer-per-hour' }).format(88));
// → 88 km/h

// Plurals: categories differ per language, so select by category
const items = new Intl.PluralRules('en-US');
const noun = { one: 'file', other: 'files' };
console.log([1, 2].map((n) => `${n} ${noun[items.select(n)]}`));
// → [ '1 file', '2 files' ]
console.log(new Intl.PluralRules('pl').select(22), new Intl.PluralRules('en', { type: 'ordinal' }).select(22));
// → few two

console.log(new Intl.RelativeTimeFormat('en', { numeric: 'auto' }).format(-1, 'day'));
// → yesterday
console.log(new Intl.ListFormat('en', { type: 'disjunction' }).format(['red', 'green', 'blue']));
// → red, green, or blue

Run on Node 24.19.0 (ICU 78.3, CLDR 48.0); identical output on Node 22.23.2 and 26.7.0.

• Formatting an instant without timeZone. new Date('2026-03-08') is UTC midnight, and new Intl.DateTimeFormat('en-US').format(d) printed 3/7/2026 in New York but 3/8/2026 with timeZone: 'UTC'.
• Calling toLocaleString(locale, options) in a hot loop. For 20,000 de-DE currency formats, a fresh toLocaleString each time took about 395 ms against about 7 ms reusing one NumberFormat (Node 24.19, one machine; the ratio is the point).
• Treating formatToParts and format as interchangeable text. In Node 22 and 24 the literal before PM came back as U+202F (narrow no-break space) from formatToParts while format() printed an ASCII space; Node 26.7 returned ASCII for both. Compare parts by type, not by raw string.
• Assuming a currency's digits. JPY has zero fraction digits and USD two; NumberFormat applies them, and a missing or malformed currency throws (TypeError or RangeError).