fix: date-util fn

.changeset/fifty-eels-applaud.md

@@ -0,0 +1,5 @@
+---
+"@zag-js/date-utils": patch
+---
+
+Fix issue where `getWeekDays` has incoherent behavior when both locale and `startOfWeekProp` are set

packages/utilities/date-utils/src/get-start-of-week.ts

@@ -1,7 +1,14 @@
-import { type DateValue, getDayOfWeek } from "@internationalized/date"
+import { type DateValue, getDayOfWeek, startOfWeek } from "@internationalized/date"
 
-export function getStartOfWeek(date: DateValue, locale: string, firstDayOfWeek: number = 0) {
-  const day = getDayOfWeek(date, locale)
-  const diff = (day - firstDayOfWeek + 7) % 7
-  return date.subtract({ days: diff })
+export function getStartOfWeek(date: DateValue, locale: string, firstDayOfWeek?: number) {
+  // If firstDayOfWeek is provided, use it; otherwise, use the locale's default
+  if (firstDayOfWeek !== undefined) {
+    // Adjust the date to the specified first day of the week
+    const currentDayOfWeek = getDayOfWeek(date, locale)
+    const daysToSubtract = (currentDayOfWeek - firstDayOfWeek + 7) % 7
+    return date.subtract({ days: daysToSubtract })
+  }
+
+  // Use the built-in startOfWeek function for locale-specific behavior
+  return startOfWeek(date, locale)
 }

packages/utilities/date-utils/tests/get-start-of-week.test.ts

@@ -0,0 +1,24 @@
+import { CalendarDate } from "@internationalized/date"
+import { describe, expect, it } from "vitest"
+import { getWeekDays } from "../src"
+
+function exec(locale: string, weekStartDay?: number) {
+  const today = new CalendarDate(2024, 8, 21)
+  const days = getWeekDays(today, weekStartDay, "UTC", locale)
+  // console.log(days.map((d) => `${d.long} ${d.value.day}`).join(","))
+  return days[0].value.day
+}
+
+describe("getStartOfWeek", () => {
+  it("en locale", () => {
+    expect(exec("en")).toBe(18)
+    expect(exec("en", 0)).toBe(18)
+    expect(exec("en", 1)).toBe(19)
+  })
+
+  it("fr locale", () => {
+    expect(exec("fr")).toBe(19)
+    expect(exec("fr", 0)).toBe(19)
+    expect(exec("fr", 1)).toBe(20)
+  })
+})