types/isLeapYear

Utility type that determines at compile time whether a year (string) is a leap year.

Signature

type TenEven = "" | "2" | "4" | "6" | "8";
type OddTen = "1" | "3" | "5" | "7" | "9";
type MultipleOf4 = `${TenEven}${"0" | "4" | "8"}` | `${OddTen}${"2" | "6"}`;
type MultipleOf100 = `${Exclude<Digit, "0">}00`;
type MultipleOf400 = `${Exclude<MultipleOf4, "0">}00`;
export type IsLeapYear<
	GenericYear extends `${number}`
> = GenericYear extends `${number | ""}${MultipleOf4}` 
	? GenericYear extends `${number | ""}${MultipleOf100}` 
		? GenericYear extends `${number | ""}${MultipleOf400}` 
			? true 
			: false 
		: true 
	: false;

Usage

This type is used in create() to reject a YYYY-02-29 date when the year is not leap.

See also

• create
• types/time