date-utils.ts

export type DayISO = `${number}-${number}-${number}`;

export function findFirstDayOfWeek(localeName: string): number {
    // weekInfo only exists on some browsers, so we default to Sunday otherwise
    // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/Locale/weekInfo

    const locale = new Intl.Locale(localeName) as Intl.Locale & {
        weekInfo?: {firstDay: number};
    }
    if (locale.weekInfo) {
        return locale.weekInfo.firstDay
    }
    return 0
}

type WeekdayInfo = {
    firstDayOfWeek: number;
    weekdayLabels: string[];
}

export function getWeekdayInfo(localeName: string): WeekdayInfo {
    const firstDayOfWeek = findFirstDayOfWeek(localeName)

    const weekdayLabelFormatter = new Intl.DateTimeFormat(localeName, {
        weekday: 'short'
    })
    const weekday = new Date(2022, 9, 2 + firstDayOfWeek) // October 2, 2022 was a Sunday
    const weekdayLabels = [...Array(7)].map(() => {
        const dayLabel = weekdayLabelFormatter.format(weekday)
        weekday.setDate(weekday.getDate() + 1)
        return dayLabel
    })

    return {
        firstDayOfWeek,
        weekdayLabels
    }
}

export function dateArgToISO(arg?: DateConstructor['arguments']): DayISO {
    if (!arg) return undefined
    if (/^\d\d\d\d-\d\d-\d\d$/g.test(arg)) return arg as DayISO
    return toISO(new Date(arg))
}

export function toISO(date: Date): DayISO {
    if (isNaN(date.getTime())) return
    return date.toISOString().slice(0, 10) as DayISO
}

export function fromISO(iso: DayISO): Date {
    return new Date(iso)
}

export function offsetISO(iso: DayISO, days: number): DayISO {
    const date = fromISO(iso)
    date.setUTCDate(date.getUTCDate() + days)
    return toISO(date)
}

export function localeOverride(locale?: string): string {
    return locale || navigator.language
}