date-fns/setDay.js

import { getDefaultOptions } from "./_lib/defaultOptions.js";
import { addDays } from "./addDays.js";
import { toDate } from "./toDate.js";

/**
 * The {@link setDay} function options.
 */

/**
 * @name setDay
 * @category Weekday Helpers
 * @summary Set the day of the week to the given date.
 *
 * @description
 * Set the day of the week to the given date.
 *
 * @typeParam DateType - The `Date` type, the function operates on. Gets inferred from passed arguments. Allows to use extensions like [`UTCDate`](https://github.com/date-fns/utc).
 * @typeParam ResultDate - The result `Date` type, it is the type returned from the context function if it is passed, or inferred from the arguments.
 *
 * @param date - The date to be changed
 * @param day - The day of the week of the new date
 * @param options - An object with options.
 *
 * @returns The new date with the day of the week set
 *
 * @example
 * // Set week day to Sunday, with the default weekStartsOn of Sunday:
 * const result = setDay(new Date(2014, 8, 1), 0)
 * //=> Sun Aug 31 2014 00:00:00
 *
 * @example
 * // Set week day to Sunday, with a weekStartsOn of Monday:
 * const result = setDay(new Date(2014, 8, 1), 0, { weekStartsOn: 1 })
 * //=> Sun Sep 07 2014 00:00:00
 */
export function setDay(date, day, options) {
  const defaultOptions = getDefaultOptions();
  const weekStartsOn =
    options?.weekStartsOn ??
    options?.locale?.options?.weekStartsOn ??
    defaultOptions.weekStartsOn ??
    defaultOptions.locale?.options?.weekStartsOn ??
    0;

  const date_ = toDate(date, options?.in);
  const currentDay = date_.getDay();

  const remainder = day % 7;
  const dayIndex = (remainder + 7) % 7;

  const delta = 7 - weekStartsOn;
  const diff =
    day < 0 || day > 6
      ? day - ((currentDay + delta) % 7)
      : ((dayIndex + delta) % 7) - ((currentDay + delta) % 7);
  return addDays(date_, diff, options);
}

// Fallback for modularized imports:
export default setDay;