Repository: date-fns/date-fns
File: src/startOfWeek/index.ts (TypeScript)

Code before:
import { LocaleOptions, WeekStartOptions } from '../types'
import toDate from '../toDate/index'
import toInteger from '../_lib/toInteger/index'
import requiredArgs from '../_lib/requiredArgs/index'

export enum Day {
  SUNDAY,
  MONDAY,
  TUESDAY,
  WEDNESDAY,
  THURSDAY,
  FRIDAY,
  SATURDAY,
}

/**
 * @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))

Review comment (suggestion):
This is clever, but there's a problem. 

First of all it will require importing enum everywhere, for example:  `nextDay(day, Dat.SUNDAY)`. Not very handy, yet explicit. Should we allow using enums (`export type Day = DayIndex | DayEnum`) as well, for type nerds? 

Secondly, as I can see it's not used and you changed your mind, but just in case, there's Day alias in types.

Code after:
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