@technobuddha/library/dist/iso-weeks-in-year.js

A large library of useful functions

import { day, daysPerWeek, month } from "./date.js";
import { isNumber } from "./is-number.js";
import { modulo } from "./modulo.js";
/**
 * Determine the number of ISO weeks within a year
 * @param input - A date within the year, or a year number
 * @param options - see {@link ISOWeeksInYearOptions}
 * @defaultValue weekOneIncludes Thursday
 * @returns The number of weeks in the year (52 or 53)
 * @group Time
 * @category Week
 */
export function isoWeeksInYear(input, { utc = false, weekOneIncludes = day.thursday } = {}) {
    const year = isNumber(input) ? input
        : utc ? input.getUTCFullYear()
            : input.getFullYear();
    const dow0 = utc ?
        new Date(Date.UTC(year, month.january, 1)).getUTCDay()
        : new Date(year, month.january, 1).getDay();
    const dow1 = utc ?
        new Date(Date.UTC(year, month.december, 31)).getUTCDay()
        : new Date(year, month.december, 31).getDay();
    const target = modulo(weekOneIncludes, daysPerWeek);
    return dow0 === target || dow1 === target ? 53 : 52;
}
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