Temporal.PlainMonthDay.prototype.monthCode

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 monthCode accessor property of Temporal.PlainMonthDay instances returns a calendar-specific string representing the month of this date. It is calendar-dependent.

Usually it is M plus a two-digit month number. For leap months, it is the previous month's code followed by L (even if it's conceptually a derivative of the following month; for example, in the Hebrew calendar, Adar I has code M05L but Adar II has code M06). If the leap month is the first month of the year, the code is M00L.

Because month is an index within a year, but PlainMonthDay doesn't have a year, there's no month property for PlainMonthDay. Therefore, monthCode is used to represent the month in a way that is independent of the year.

The set accessor of monthCode is undefined. You cannot change this property directly. Use the with() method to create a new Temporal.PlainMonthDay object with the desired new value.

For general information and more examples, see Temporal.PlainDate.prototype.monthCode.

Examples

Using monthCode

const md = Temporal.PlainMonthDay.from("07-01"); // ISO 8601 calendar
console.log(md.monthCode); // "M07"

const md2 = Temporal.PlainMonthDay.from("2021-05-01[u-ca=chinese]");
console.log(md2.monthCode); // "M03"

const md3 = Temporal.PlainMonthDay.from("2023-04-01[u-ca=chinese]");
console.log(md3.monthCode); // "M02L"

Changing monthCode

const md = Temporal.PlainMonthDay.from("07-01");
const newMD = md.with({ monthCode: "M03" });
console.log(newMD.toString()); // 03-01

For other calendars, as long as there exists a year in which the month-day is valid, the month-day is considered valid, and the underlying reference year may therefore change. For example:

const md = Temporal.PlainMonthDay.from({
  monthCode: "M02",
  day: 30,
  calendar: "hebrew",
});
console.log(md.toString()); // 1971-11-18[u-ca=hebrew]
console.log(md.toLocaleString("en-US", { calendar: "hebrew" })); // 30 Heshvan
// 30 Heshvan only exists in 1971, but this year is not a leap year
const newMD = md.with({ monthCode: "M05L" });
console.log(newMD.toString()); // 1970-03-08[u-ca=hebrew]
console.log(newMD.toLocaleString("en-US", { calendar: "hebrew" })); // 30 Adar I

Specifications

Specification
Temporal # sec-get-temporal.plainmonthday.prototype.monthcode

Browser compatibility

	Desktop					Mobile						Server
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	WebView Android	Deno	Node.js
`monthCode`	No	No	139	No	No	No	139	No	No	No	No	1.40	No

Examples

Using monthCode

Changing monthCode

Specifications

Browser compatibility

See also