Locale-aware number and currency formatting
Intl.NumberFormat
Chrome24
Edge12
Firefox29
Safari10
Features it needs
- IntlWidely available
These libraries hardcode their own thousands separators, currency symbols, and rounding rules, and most ship a single locale by default. Intl.NumberFormat is the same formatting logic the operating system already has: pass a locale and a style, currency, percent, or unit, and it places the symbol, separator, and decimal grouping the way that locale actually expects, with no formatting rules to maintain.
When this applies
Formatting a number as currency, a percentage, or a locale-correct thousands-grouped number for display.
The native approach
new Intl.NumberFormat("en-US", {
style: "currency",
currency: "USD",
}).format(1234.5); // "$1,234.50"When the dependency is still right
An answer that always says "the platform covers it" is worse than no answer. These are the cases where this one does not hold.
- You format units or compact notation. style: "unit" and notation: "compact" arrived long after the rest of Intl.NumberFormat, in Chrome 77, Firefox 78 and Safari 14.1, so an older target does not get them.
- You need safe decimal arithmetic, such as adding money values without floating-point rounding errors. currency.js and accounting.js do that math for you; Intl.NumberFormat only formats a number you already computed correctly.
- You need to parse a formatted string back into a number. Intl.NumberFormat is format-only; these libraries often provide the reverse direction too.
- You format byte sizes and need the unit chosen for you. Intl.NumberFormat formats a magnitude you already picked, so "kilobyte" on 1200 prints 1,200 kB rather than 1.2 MB, and dividing down to the right unit first is the part pretty-bytes and filesize actually do.
- You need binary units. The sanctioned unit list has no kibibyte, mebibyte or gibibyte, and asking for one throws a RangeError rather than falling back, so anything reporting KiB or MiB keeps its library.
- You need a custom format pattern, such as a specific abbreviation style, that the options Intl.NumberFormat exposes cannot express.
Signs it was hand-rolled
No package is involved in any of these, so nothing would match in a package.json. If the code looks like one of them, this rule applies anyway, and the conditions above still decide.
- a regex inserting thousands separators into a number's string form
- toFixed followed by string surgery to add a currency symbol and group the digits