Number Formatter (Intl.NumberFormat)

A visual front-end for Intl.NumberFormat. Pick a locale (en-US, de-DE, ja-JP, …), a style (decimal, currency, percent, unit), a notation (standard, scientific, engineering, compact), and digit precision. The tool shows the formatted output and the exact JavaScript call that produced it, so you can paste it into your codebase. Helpful for previewing how prices, counts, percentages, or unit values will render in different markets.

The separators swapping is the change people notice first and the one that breaks parsing quietly. German writes 1.234,56 where English writes 1,234.56, so a number formatted for one locale and read back by code expecting the other is off by three orders of magnitude or fails outright. Formatting is one-way: Intl has no parser, and recovering a number from a localised string means stripping the separators for that locale yourself.

Grouping is not always in threes. Indian locales group the lowest three digits and then in pairs, so ten million reads as 1,00,00,000, and several locales group only above a threshold or not at all. Any layout that reserved space by counting commas has assumed something the standard does not promise.

Currency formatting carries the position, the symbol and the digit count, all of which vary. The euro follows the amount in German and precedes it in Irish English, the yen has no minor unit so it formats with zero decimals, and the dinar has three. Hard-coding two decimal places is the assumption that breaks at both ends of that list.

Compact notation is locale-specific text rather than an abbreviation you can localise later. English gives 1.2M, German 1,2 Mio., Japanese 120万 on a different grouping entirely, because the language counts in ten-thousands. A design that budgeted four characters for a compact number is sized for English alone.

The browser supplies the data, so output can differ slightly between engines and versions as CLDR is updated. Pin what matters to a test rather than to a screenshot.