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diff --git a/date-fns/src/startOfWeek/index.ts b/date-fns/src/startOfWeek/index.ts
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+import { LocaleOptions, WeekStartOptions } from '../types'
+import toDate from '../toDate/index'
+import toInteger from '../_lib/toInteger/index'
+import requiredArgs from '../_lib/requiredArgs/index'
+
+/**
+ * @name startOfWeek
+ * @category Week Helpers
+ * @summary Return the start of a week for the given date.
+ *
+ * @description
+ * Return the start of a week for the given date.
+ * The result will be in the local timezone.
+ *
+ * ### v2.0.0 breaking changes:
+ *
+ * - [Changes that are common for the whole library](https://github.com/date-fns/date-fns/blob/master/docs/upgradeGuide.md#Common-Changes).
+ *
+ * @param {Date|Number} date - the original date
+ * @param {Object} [options] - an object with options.
+ * @param {Locale} [options.locale=defaultLocale] - the locale object. See [Locale]{@link https://date-fns.org/docs/Locale}
+ * @param {0|1|2|3|4|5|6} [options.weekStartsOn=0] - the index of the first day of the week (0 - Sunday)
+ * @returns {Date} the start of a week
+ * @throws {TypeError} 1 argument required
+ * @throws {RangeError} `options.weekStartsOn` must be between 0 and 6
+ *
+ * @example
+ * // The start of a week for 2 September 2014 11:55:00:
+ * const result = startOfWeek(new Date(2014, 8, 2, 11, 55, 0))
+ * //=> Sun Aug 31 2014 00:00:00
+ *
+ * @example
+ * // If the week starts on Monday, the start of the week for 2 September 2014 11:55:00:
+ * const result = startOfWeek(new Date(2014, 8, 2, 11, 55, 0), { weekStartsOn: 1 })
+ * //=> Mon Sep 01 2014 00:00:00
+ */
+export default function startOfWeek(
+  dirtyDate: Date | number,
+  dirtyOptions?: LocaleOptions & WeekStartOptions
+): Date {
+  requiredArgs(1, arguments)
+
+  const options = dirtyOptions || {}
+  const locale = options.locale
+  const localeWeekStartsOn =
+    locale && locale.options && locale.options.weekStartsOn
+  const defaultWeekStartsOn =
+    localeWeekStartsOn == null ? 0 : toInteger(localeWeekStartsOn)
+  const weekStartsOn =
+    options.weekStartsOn == null
+      ? defaultWeekStartsOn
+      : toInteger(options.weekStartsOn)
+
+  // Test if weekStartsOn is between 0 and 6 _and_ is not NaN
+  if (!(weekStartsOn >= 0 && weekStartsOn <= 6)) {
+    throw new RangeError('weekStartsOn must be between 0 and 6 inclusively')
+  }
+
+  const date = toDate(dirtyDate)
+  const day = date.getDay()
+  const diff = (day < weekStartsOn ? 7 : 0) + day - weekStartsOn
+
+  date.setDate(date.getDate() - diff)
+  date.setHours(0, 0, 0, 0)
+  return date
+}