number Optional

A Number or BigInt to format.

An Array of objects containing the formatted number in parts.

The formatToParts() method is useful for custom formatting of number strings. It returns an Array of objects containing the locale-specific tokens from which it possible to build custom strings while preserving the locale-specific parts. The structure the formatToParts() method returns, looks like this:

[
  { type: "integer", value: "3" },
  { type: "group", value: "." },
  { type: "integer", value: "500" }
]

Possible types are the following:

currency

The currency string, such as the symbols "$" and "€" or the name "Dollar", "Euro" depending on how currencyDisplay is specified.

decimal

The decimal separator string (".").

fraction

The fraction number.

group

The group separator string (",").

infinity

The Infinity string ("∞").

integer

The integer number.

literal

Any literal strings or whitespace in the formatted number.

minusSign

The minus sign string ("-").

nan

The NaN string ("NaN").

plusSign

The plus sign string ("+").

percentSign

The percent sign string ("%").

unit

The unit string, such as the "l" or "litres", depending on how unitDisplay is specified.

NumberFormat outputs localized, opaque strings that cannot be manipulated directly:

const number = 3500;

const formatter = new Intl.NumberFormat('de-DE', {
  style: 'currency',
  currency: 'EUR'
});

formatter.format(number);
// "3.500,00 €"

However, in many User Interfaces there is a desire to customize the formatting of this string. The formatToParts method enables locale-aware formatting of strings produced by NumberFormat formatters by providing you the string in parts:

formatter.formatToParts(number);

// return value:
[
  { type: "integer",  value: "3"   },
  { type: "group",    value: "."   },
  { type: "integer",  value: "500" },
  { type: "decimal",  value: ","   },
  { type: "fraction", value: "00"  },
  { type: "literal",  value: " "   },
  { type: "currency", value: "€"   }
]

Now the information is available separately and it can be formatted and concatenated again in a customized way. For example by using Array.prototype.map(), arrow functions, a switch statement, template literals, and Array.prototype.reduce().

const numberString = formatter.formatToParts(number).map(({type, value}) => {
  switch (type) {
    case 'currency': return `<strong>${value}</strong>`;
    default : return value;
  }
}).reduce((string, part) => string + part);

This will make the currency bold, when using the formatToParts() method.

console.log(numberString);
// "3.500,00 <strong>€</strong>"

• Intl.NumberFormat
• Intl.NumberFormat.prototype.format
• Formatting dates: Intl.DateTimeFormat.prototype.formatToParts()