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

js
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

js
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:

js
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 proposal
# sec-get-temporal.plainmonthday.prototype.monthcode

Browser compatibility

Report problems with this compatibility data on GitHub
desktopmobileserver
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
Deno
Node.js
monthCode
Experimental

Legend

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No support
No support
Experimental. Expect behavior to change in the future.
See implementation notes.
User must explicitly enable this feature.

See also