i18n/intl-format

Why

Date order, separators, month names, decimal marks, grouping and the place of the currency sign differ by locale: 11/12/2026 is November in the US and December in Poland, and "1,234.50 zł" is wrong in Polish ("1234,50 zł"). Hand-built strings are right for one locale at most.

Fix

Create formatters once per locale and reuse them: new Intl.DateTimeFormat(locale, { dateStyle: 'medium' }), new Intl.NumberFormat(locale, { style: 'currency', currency }), Intl.RelativeTimeFormat for "3 days ago", Intl.ListFormat for "A, B and C". Keep machine formats (ISO 8601 in datetime, APIs) separate from display.

Examples

// bad
d.getDate() + '.' + (d.getMonth() + 1) + '.' + d.getFullYear()
total.toFixed(2) + ' zł'
// good
new Intl.DateTimeFormat(locale, { dateStyle: 'medium' }).format(d)
new Intl.NumberFormat(locale, { style: 'currency', currency: 'PLN' }).format(total)

Sources

• MDN, Intl.DateTimeFormat
• MDN, Intl.NumberFormat

Read it in the terminal

npx github:dawidolko/frontend-skills explain i18n/intl-format