JavaScript’s Intl namespace (ECMA-402) helps you localize your app, no extra libraries required. It formats numbers, dates, lists, and more according to local rules.

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Format Numbers with Intl.NumberFormat

Numbers look different across regions — commas, decimals, and currency placement all vary. Intl.NumberFormat handles this automatically:

const number = 1234567.89;
new Intl.NumberFormat().format(number); 
// "1,234,567.89" in en-US

new Intl.NumberFormat('de-DE').format(number); 
// "1.234.567,89"

new Intl.NumberFormat('fr-FR', { style: 'currency', currency: 'EUR' })
  .format(42.5); 
// "42,50 €"

Use this to format numbers, currencies, and percentages in a way that feels natural to users worldwide.

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Format Lists with Intl.ListFormat

Languages differ in how they join items. English: “A, B, and C.” French: “A, B et C.” Intl.ListFormat handles this:

const fruits = ['apples', 'bananas', 'cherries'];

new Intl.ListFormat().format(fruits); 
// "apples, bananas, and cherries"

Use this to join arrays into natural-sounding lists for any language.

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Format Dates with Intl.DateTimeFormat

Dates and times vary even more — order, time zones, 12h vs 24h. Intl.DateTimeFormat makes it consistent:

const date = new Date('2025-09-23T15:30:00Z');

new Intl.DateTimeFormat('en-US').format(date);  
// "9/23/2025"

new Intl.DateTimeFormat('en-GB').format(date);  
// "23/09/2025"

new Intl.DateTimeFormat('fr-FR', { dateStyle: 'full', timeStyle: 'short' })
  .format(date);
// "mardi 23 septembre 2025 à 17:30"

Format dates and times for any locale, including custom styles and time zones.

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Format Relative Time with Intl.RelativeTimeFormat

For “in 2 hours” or “yesterday,” use Intl.RelativeTimeFormat. It lets you express differences relative to now:

const rtf = new Intl.RelativeTimeFormat('en', { numeric: 'auto' });

rtf.format(-1, 'day');  
// "yesterday"

rtf.format(2, 'hour');  
// "in 2 hours"

Create phrases like “yesterday” or “in 2 hours” that adapt to the user’s language.

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Handle Plurals with Intl.PluralRules

Different languages form plurals differently. Intl.PluralRules gives you the correct plural category:

const pr = new Intl.PluralRules('en-US');

pr.select(1); // "one"
pr.select(5); // "other"

// Example usage:
const messages = {
  one: "1 apple",
  other: "{count} apples"
};
const count = 5;
messages[pr.select(count)].replace("{count}", count);
// "5 apples"

Get the right plural form for numbers in any language.

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Display Locale Info with Intl.DisplayNames

Intl.DisplayNames converts locale codes or region codes into human-friendly names:

const dn = new Intl.DisplayNames(['en'], { type: 'language' });

dn.of('fr'); // "French"
dn.of('en-US'); // "American English"

const regions = new Intl.DisplayNames(['en'], { type: 'region' });
regions.of('DE'); // "Germany"

Convert language and region codes into readable names for users.

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Sort with Intl.Collator

Intl.Collator lets you compare and sort strings in a way that respects language rules (accents, case, etc.).

const items = ['zebra', 'äpfel', 'apple'];
items.sort(new Intl.Collator('de').compare);
// [ 'äpfel', 'apple', 'zebra' ]

Use this for sorting lists so results feel natural to users in any locale.

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Split Text with Intl.Segmenter

Intl.Segmenter breaks text into words, sentences, or graphemes, following language-specific rules.

const segmenter = new Intl.Segmenter('en', { granularity: 'word' });
const input = 'Hello world!';
const words = Array.from(segmenter.segment(input), s => s.segment);
// [ 'Hello', ' ', 'world', '!' ]

Great for tokenizing text, highlighting, or advanced text processing.

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