current-week-number/index.js

/**
 * current-week-number <https://github.com/datetime/current-week-number>
 *
 * Copyright (c) 2014-2015 Charlike Mike Reagent, contributors.
 * Released under the MIT license.
 */

'use strict';

/**
 * Get week number of the current date/year or given valid `Date` string format
 *
 * **Example:**
 *
 * ```js
 * var currentWeekNumber = require('current-week-number');
 *
 * // june 27, 2014
 * currentWeekNumber();
 * //=> 26
 *
 * currentWeekNumber('March 24, 2015');
 * //=> 13
 *
 * currentWeekNumber(new Date('March 24, 2015'));
 * //=> 13
 *
 * currentWeekNumber('03/24/2016');
 * //=> 12, cuz' year is leap
 *
 * currentWeekNumber('August 07, 2015');
 * //=> 32
 *
 * currentWeekNumber(new Date('August 07, 2016'));
 * //=> 31
 *
 * currentWeekNumber('02/16/2015');
 * //=> 8
 * ```
 *
 * @name currentWeekNumber
 * @param  {String} `[date]` every valid Date-ish string format
 * @return {Number}
 * @api public
 */
module.exports = function currentWeekNumber(date) {
  var instance;

  if (typeof date === 'string' && date.length) {
    instance = new Date(date);
  } else if (date instanceof Date) {
    instance = date;
  } else {
    instance = new Date();
  }

  // Create a copy of this date object
  var target = new Date(instance.valueOf());

  // ISO week date weeks start on monday
  // so correct the day number
  var dayNr = (instance.getDay() + 6) % 7;

  // ISO 8601 states that week 1 is the week
  // with the first thursday of that year.
  // Set the target date to the thursday in the target week
  target.setDate(target.getDate() - dayNr + 3);

  // Store the millisecond value of the target date
  var firstThursday = target.valueOf();

  // Set the target to the first thursday of the year
  // First set the target to january first
  target.setMonth(0, 1);
  // Not a thursday? Correct the date to the next thursday
  if (target.getDay() !== 4) {
    target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7);
  }

  // The weeknumber is the number of weeks between the
  // first thursday of the year and the thursday in the target week
  var weekNumber = 1 + Math.ceil((firstThursday - target) / 604800000);
  return weekNumber;
};