Intl

JavaScript · Reference cheat sheet

📋 Overview

The Intl namespace provides locale-aware formatting and comparison: dates, numbers, lists, relative time, plurals, and collation. Prefer Intl over hand-rolled locale strings for i18n correctness and accessibility.

🔧 Core concepts

Constructor	Use
`Intl.DateTimeFormat`	dates/times
`Intl.NumberFormat`	numbers/currency/units
`Intl.RelativeTimeFormat`	“3 days ago”
`Intl.ListFormat`	“A, B, and C”
`Intl.PluralRules`	plural categories
`Intl.Collator`	sort / compare strings
`Intl.DisplayNames`	language/region names
`Intl.Segmenter`	grapheme/word segments

Locales: BCP 47 tags — "en", "en-US", "de-DE", navigator.language.
Options: style, currency, timeZone, numberingSystem, etc.
supportedLocalesOf: feature detection for locale lists.

new Intl.NumberFormat("en-US", {
  style: "currency",
  currency: "USD",
}).format(1234.5); // "$1,234.50"

💡 Examples

const locale = navigator.language || "en";

// Dates
new Intl.DateTimeFormat(locale, {
  dateStyle: "medium",
  timeStyle: "short",
}).format(new Date());

// Time zone
new Intl.DateTimeFormat("en-GB", {
  timeZone: "UTC",
  timeStyle: "long",
}).format(new Date());

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

// Lists
new Intl.ListFormat(locale, { style: "long", type: "conjunction" }).format([
  "red",
  "green",
  "blue",
]);

// Sort
const collator = new Intl.Collator(locale, { sensitivity: "base" });
["ä", "a", "z"].sort(collator.compare);

// Plurals
const pr = new Intl.PluralRules("ru");
pr.select(3); // "few" etc.

// Display names
new Intl.DisplayNames("en", { type: "language" }).of("fr"); // "French"

// Compact notation
new Intl.NumberFormat("en", {
  notation: "compact",
  compactDisplay: "short",
}).format(1_200_000); // "1.2M"

⚠️ Pitfalls

Locale data availability varies by engine — test critical locales.
Don’t concatenate translated fragments — format whole messages (use ICU MessageFormat libs for complex cases).
Date#toLocaleString is convenient but less reusable than a shared DateTimeFormat instance.
Currency requires ISO codes (USD), not symbols alone.
Sorting without Collator breaks non-English languages.

date.md — Date object
number.md — numeric values
strings.md — string compare
template_literals.md — building messages
performance.md — reuse formatters

Intl

Intl

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

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