18 lines
939 B
JavaScript
18 lines
939 B
JavaScript
import { getWeekStart } from '#tofu/tools/dates/getWeekStart.js'
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const WEEK_IN_MS = 7 * 24 * 60 * 60 * 1000
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// Returns the ISO 8601 week identifier of `date`, for example "2026-W38".
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// ISO 8601 rule: a week belongs to the year of its Thursday, so the week-year
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// can differ from the calendar year around January 1st.
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export function getWeekId(date) {
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const thursday = getWeekStart(date)
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thursday.setDate(thursday.getDate() + 3)
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const weekYear = thursday.getFullYear()
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// January 4th is always in the first ISO week, so its Thursday is the first Thursday.
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const firstThursday = getWeekStart(new Date(weekYear, 0, 4))
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firstThursday.setDate(firstThursday.getDate() + 3)
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// Rounding absorbs the one hour a daylight saving change adds to or removes from the span.
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const weekNumber = 1 + Math.round((thursday.getTime() - firstThursday.getTime()) / WEEK_IN_MS)
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return `${weekYear}-W${String(weekNumber).padStart(2, '0')}`
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}
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