Temporal.PlainYearMonth.compare()

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The Temporal.PlainYearMonth.compare() static method returns a number (-1, 0, or 1) indicating whether the first year-month comes before, is the same as, or comes after the second year-month. It is equivalent to comparing their underlying ISO 8601 dates. Two year-months from different calendars may be considered equal if they start on the same ISO date.

Note: PlainYearMonth objects keep track of a reference ISO day, which is also used in the comparison. This day is automatically set when using the Temporal.PlainYearMonth.from() method, but can be set manually using the Temporal.PlainYearMonth() constructor, causing two equivalent year-months to be considered different if they have different reference days. For this reason, you should avoid using the constructor directly and prefer the from() method.

Syntax

js
Temporal.PlainYearMonth.compare(yearMonth1, yearMonth2)

Parameters

yearMonth1

A string, an object, or a Temporal.PlainYearMonth instance representing the first year-month to compare. It is converted to a Temporal.PlainYearMonth object using the same algorithm as Temporal.PlainYearMonth.from().

yearMonth2

The second year-month to compare, converted to a Temporal.PlainYearMonth object using the same algorithm as yearMonth1.

Return value

Returns -1 if yearMonth1 comes before yearMonth2, 0 if they are the same, and 1 if yearMonth2 comes after. They are compared by their underlying date values (usually the first day of the month), ignoring their calendars.

Examples

Using Temporal.PlainYearMonth.compare()

js
const ym1 = Temporal.PlainYearMonth.from("2021-08");
const ym2 = Temporal.PlainYearMonth.from("2021-09");
console.log(Temporal.PlainYearMonth.compare(ym1, ym2)); // -1

const ym3 = Temporal.PlainYearMonth.from("2021-07");
console.log(Temporal.PlainYearMonth.compare(ym1, ym3)); // 1

Comparing year-months in different calendars

js
const ym1 = Temporal.PlainYearMonth.from({ year: 2021, month: 8 });
const ym2 = Temporal.PlainYearMonth.from({
  year: 2021,
  month: 8,
  calendar: "islamic",
});
const ym3 = Temporal.PlainYearMonth.from({
  year: 2021,
  month: 8,
  calendar: "hebrew",
});
console.log(ym1.toString()); // "2021-08"
console.log(ym2.toString()); // "2582-12-18[u-ca=islamic]"
console.log(ym3.toString()); // "-001739-04-06[u-ca=hebrew]"
console.log(Temporal.PlainYearMonth.compare(ym1, ym2)); // -1
console.log(Temporal.PlainYearMonth.compare(ym1, ym3)); // 1

Sorting an array of year-months

The purpose of this compare() function is to act as a comparator to be passed to Array.prototype.sort() and related functions.

js
const months = [
  Temporal.PlainYearMonth.from({ year: 2021, month: 8 }),
  Temporal.PlainYearMonth.from({
    year: 2021,
    month: 8,
    calendar: "islamic",
  }),
  Temporal.PlainYearMonth.from({ year: 2021, month: 8, calendar: "hebrew" }),
];

months.sort(Temporal.PlainYearMonth.compare);
console.log(months.map((d) => d.toString()));
// [ "-001739-04-06[u-ca=hebrew]", "2021-08", "2582-12-18[u-ca=islamic]" ]

Specifications

Specification
Temporal proposal
# sec-temporal.plainyearmonth.compare

Browser compatibility

BCD tables only load in the browser

See also