Intl.NumberFormat, dates & localeCompare

Format for people

Lesson 18 of 20 · 10 min · Module 5: Real-world data

1234.5 is "$1,234.50" for an American, "1.234,50 €" for a German, and names like "Émile" should sort next to "Emma", not after "Zoe". The built-in Intl API handles these rules for every language.

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By the end, you’ll be able to…

Format currency for different countries
Compare text the way people read it
Format dates without time-zone shifts

The idea

new Intl.NumberFormat(locale, { style: "currency", currency }) formats money. Intl.DateTimeFormat formats dates. a.localeCompare(b, locale, { sensitivity: "base" }) compares text the way people expect, ignoring case and accents.

const eur = new Intl.NumberFormat("de-DE", { style: "currency", currency: "EUR" });
console.log(eur.format(1234.5));
console.log(["b", "a", "C"].sort((x, y) => x.localeCompare(y)));

01Locales carry rules

The locale decides the decimal separator, the grouping symbol, where the currency sign goes, and even the spaces used. Building such strings by hand is error-prone for every country but your own.

02Default sorting is by code points

["b", "a", "C"].sort() gives ["C", "a", "b"] because uppercase letters have lower codes. localeCompare with sensitivity "base" treats a, A and á as equal letters.

console.log(["b", "a", "C"].sort());
console.log(["b", "a", "C"].sort((x, y) => x.localeCompare(y)));

Output

["C","a","b"]
["a","b","C"]

03Pin the time zone

Formatting midnight UTC in a time zone west of UTC shows the previous day. timeZone: "UTC" formats the calendar date exactly as stored.

Your turn

Write formatPrice(cents, currency = "USD", locale = "en-US") that formats an amount given in cents as currency. Write sortNames(names, locale = "en") that returns a new, alphabetically sorted array, ignoring case and accents, without changing the input. Write formatDate(iso, locale = "en-GB") that turns "2026-10-20" into "20 October 2026" (day numeric, month long, year numeric, in UTC).

What the checks look for

Formats US dollars
Formats euros for German readers
Sorts names ignoring case and accents, without changing the input
Formats a date in words

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Take this with you

Let Intl handle language rules instead of hand-made formatting.

Go a little further

Use Intl.RelativeTimeFormat to show "in 3 days" or "2 days ago" for a deadline.