Intl API

Also Known As

TL;DR

Explanation

The Intl API provides: Intl.NumberFormat (numbers, currencies, percentages), Intl.DateTimeFormat (dates, times with locale-aware formatting), Intl.RelativeTimeFormat ('2 days ago', 'in 3 hours'), Intl.ListFormat ('A, B, and C'), Intl.Collator (locale-aware string sorting), Intl.PluralRules (correct pluralisation per locale), and Intl.Segmenter (word/sentence boundaries). All major browsers support it. It handles locale-specific rules like Arabic RTL, German ordinal numbers, Japanese date formats, and Indian number grouping.

Common Misconception

Why It Matters

Common Mistakes

• Creating a new Intl.NumberFormat on every call in a loop — formatters are expensive to construct; cache them.
• Not using navigator.language for the user's locale — hardcoding 'en-US' breaks for international users.
• Using Intl.RelativeTimeFormat without specifying the unit — 'days', 'hours' must be explicit.
• Forgetting Intl.Collator for sorting strings — standard < operator gives wrong order for accented characters.

Code Examples

// Manual formatting — locale-unaware:
const price = '$' + amount.toFixed(2); // Wrong for non-US users
const date = month + '/' + day + '/' + year; // Ambiguous internationally

// Wrong sort order for accented chars:
words.sort((a, b) => a < b ? -1 : 1); // Wrong for 'café' vs 'can'

// Intl API — automatic locale handling:
const price = new Intl.NumberFormat(navigator.language, {
    style: 'currency', currency: 'USD'
}).format(amount); // '1,234.56' (US) or '1.234,56 $' (DE)

const date = new Intl.DateTimeFormat(navigator.language, {
    year: 'numeric', month: 'long', day: 'numeric'
}).format(new Date()); // 'March 16, 2026' or '16. März 2026'

const relative = new Intl.RelativeTimeFormat(navigator.language)
    .format(-2, 'day'); // '2 days ago' or 'il y a 2 jours'

words.sort(new Intl.Collator(navigator.language).compare); // Correct sort

References

• ↗ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl

Tags

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