date Optional

The date to format.

An Array of objects containing the formatted date in parts.

The formatToParts() method is useful for custom formatting of date 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: "day", value: "17" },
  { type: "weekday", value: "Monday" },
];

Possible types are the following:

day

The string used for the day, for example "17".

dayPeriod

The string used for the day period, for example, "AM", "PM", "in the morning", or "noon"

era

The string used for the era, for example "BC" or "AD".

fractionalSecond

The string used for the fractional seconds, for example "0" or "00" or "000".

hour

The string used for the hour, for example "3" or "03".

literal

The string used for separating date and time values, for example "/", ",", "o'clock", "de", etc.

minute

The string used for the minute, for example "00".

month

The string used for the month, for example "12".

relatedYear

The string used for the related 4-digit Gregorian year, in the event that the calendar's representation would be a yearName instead of a year, for example "2019".

second

The string used for the second, for example "07" or "42".

timeZone

The string used for the name of the time zone, for example "UTC". Default is the timezone of the current environment.

weekday

The string used for the weekday, for example "M", "Monday", or "Montag".

year

The string used for the year, for example "2012" or "96".

yearName

The string used for the yearName in relevant contexts, for example "geng-zi"

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

const date = Date.UTC(2012, 11, 17, 3, 0, 42);

const formatter = new Intl.DateTimeFormat("en-us", {
  weekday: "long",
  year: "numeric",
  month: "numeric",
  day: "numeric",
  hour: "numeric",
  minute: "numeric",
  second: "numeric",
  fractionalSecondDigits: 3,
  hour12: true,
  timeZone: "UTC",
});

formatter.format(date);
// "Monday, 12/17/2012, 3:00:42.000 AM"

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 DateTimeFormat formatters by providing you the string in parts:

formatter.formatToParts(date);

// return value:
[
  { type: "weekday", value: "Monday" },
  { type: "literal", value: ", " },
  { type: "month", value: "12" },
  { type: "literal", value: "/" },
  { type: "day", value: "17" },
  { type: "literal", value: "/" },
  { type: "year", value: "2012" },
  { type: "literal", value: ", " },
  { type: "hour", value: "3" },
  { type: "literal", value: ":" },
  { type: "minute", value: "00" },
  { type: "literal", value: ":" },
  { type: "second", value: "42" },
  { type: "fractionalSecond", value: "000" },
  { type: "literal", value: " " },
  { type: "dayPeriod", value: "AM" },
];

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.join().

const dateString = formatter
  .formatToParts(date)
  .map(({ type, value }) => {
    switch (type) {
      case "dayPeriod":
        return `<em>${value}</em>`;
      default:
        return value;
    }
  })
  .join("");

This will emphasize the day period when using the formatToParts() method.

console.log(formatter.format(date));
// "Monday, 12/17/2012, 3:00:42.000 AM"

console.log(dateString);
// "Monday, 12/17/2012, 3:00:42.000 <em>AM</em>"

In some cases, calendars use named years. Chinese and Tibetan calendars, for example, use a 60-year sexagenary cycle of named years. These years are disambiguated by relationship to corresponding years on the Gregorian calendar. When this is the case, the result of formatToParts() will contain an entry for relatedYear when a year would normally be present, containing the 4-digit Gregorian year, instead of an entry for year. Setting an entry in the bag for year (with any value) will yield both the and the yearName Gregorian relatedYear:

const opts = { year: "numeric", month: "numeric", day: "numeric" };
const df = new Intl.DateTimeFormat("zh-u-ca-chinese", opts);
df.formatToParts(Date.UTC(2012, 11, 17, 3, 0, 42));

// return value
[
  { type: "relatedYear", value: "2012" },
  { type: "literal", value: "年" },
  { type: "month", value: "十一月" },
  { type: "day", value: "4" },
];

If the year option is not set in the bag (to any value), the result will include only the relatedYear:

const df = new Intl.DateTimeFormat("zh-u-ca-chinese");
df.formatToParts(Date.UTC(2012, 11, 17, 3, 0, 42));

// return value
[
  { type: "relatedYear", value: "2012" },
  { type: "literal", value: "年" },
  { type: "month", value: "十一月" },
  { type: "day", value: "4" },
];

In cases where the year would be output, .format() may commonly present these side-by-side:

const df = new Intl.DateTimeFormat("zh-u-ca-chinese", { year: "numeric" });
df.format(Date.UTC(2012, 11, 17, 3, 0, 42)); // 2012壬辰年

This also makes it possible to mix locale and calendar in both format:

const df = new Intl.DateTimeFormat("en-u-ca-chinese", { year: "numeric" });
const date = Date.UTC(2012, 11, 17, 3, 0, 42);
df.format(date); // 2012(ren-chen)

And formatToParts:

const opts = { month: "numeric", day: "numeric", year: "numeric" };
const df = new Intl.DateTimeFormat("en-u-ca-chinese", opts);
const date = Date.UTC(2012, 11, 17, 3);
df.formatToParts(date);
// [
//   { type: 'month', value: '11' },
//   { type: 'literal', value: '/' },
//   { type: 'day', value: '4' },
//   { type: 'literal', value: '/' },
//   { type: 'relatedYear', value: '2012' }
// ]

• Intl.DateTimeFormat
• Intl.DateTimeFormat.prototype.format()
• Date.prototype.toLocaleString()
• Date.prototype.toLocaleDateString()
• Date.prototype.toLocaleTimeString()