「Week-Numbering Year Helpers」：date-fnsライブラリで週番号に基づいた年始を計算

What is startOfWeekYear?

The startOfWeekYear function in "date-fns" calculates and returns the date that marks the beginning of the week-numbering year for a given date. The week-numbering year is a specialized year concept that aligns with the ISO week numbering system, where weeks start on Monday and the first week of the year contains at least one day in January.

How does startOfWeekYear work?

The startOfWeekYear function takes two primary arguments:

Date
The date for which you want to determine the start of the week-numbering year.
Options (optional)
An object containing options that customize the calculation of the week-numbering year.
- weekStartsOn: An integer between 0 and 6, specifying the day of the week that is considered the first day of the week. The default is 0 (Sunday).
- firstWeekContainsDate: An integer between 1 and 7, specifying the day of January that must be included in the first week of the year. The default is 1.

Determine the week-numbering year
Use the getWeekYear function from "date-fns" to identify the week-numbering year associated with the given date.
Set the initial date
Create a new date object using the given date but set the day, month, and year to their minimum values (e.g., January 1st of the same year).
Adjust the date to the week-numbering year
Set the full year of the initial date to the previously determined week-numbering year.
Apply options
If provided, adjust the date based on the weekStartsOn and firstWeekContainsDate options.
- For weekStartsOn, subtract the value from the day of the week to ensure the desired first day of the week.
- For firstWeekContainsDate, set the day of the month to the specified value.
Return the start date
The resulting date represents the start of the week-numbering year for the given date and the specified options.

Example Usage

import startOfWeekYear from 'date-fns';

// Get the start of the week-numbering year for July 2, 2005 (default options)
const startOfWeekYearDate = startOfWeekYear(new Date(2005, 6, 2));
console.log(startOfWeekYearDate); // Output: Sun Dec 26 2004 00:00:00

// Get the start of the week-numbering year for July 2, 2005, considering Monday as the first day of the week and January 4th as the first week date
const modifiedStartOfWeekYearDate = startOfWeekYear(new Date(2005, 6, 2), {
  weekStartsOn: 1,
  firstWeekContainsDate: 4
});
console.log(modifiedStartOfWeekYearDate); // Output: Mon Jan 03 2005 00:00:00

Key Points

The function allows optional customization of the week start day and the first week date using the options object.
It calculates the date that marks the beginning of the week-numbering year for a given date, considering the ISO week numbering system.
The startOfWeekYear function is part of the "date-fns" library and specifically belongs to the "Week-Numbering Year Helpers" category.

Calculating the Start of Week-Numbering Year for Various Dates

import startOfWeekYear from 'date-fns';

const dates = [
  new Date(2024, 0, 1), // January 1, 2024
  new Date(2024, 6, 2), // July 2, 2024
  new Date(2024, 11, 31), // December 31, 2024
  new Date(2025, 1, 1), // January 1, 2025
  new Date(2025, 6, 2), // July 2, 2025
  new Date(2025, 12, 31), // December 31, 2025
];

for (const date of dates) {
  const startOfWeekYearDate = startOfWeekYear(date);
  console.log(`Start of week-numbering year for ${date.toDateString()}: ${startOfWeekYearDate.toDateString()}`);
}

Handling Different Week Start Days

import startOfWeekYear from 'date-fns';

const dates = [
  new Date(2024, 0, 1), // January 1, 2024
  new Date(2024, 6, 2), // July 2, 2024
  new Date(2024, 11, 31), // December 31, 2024
  new Date(2025, 1, 1), // January 1, 2025
  new Date(2025, 6, 2), // July 2, 2025
  new Date(2025, 12, 31), // December 31, 2025
];

const weekStartDays = [0, 1, 2, 3, 4, 5, 6]; // Sunday to Saturday

for (const date of dates) {
  for (const weekStartDay of weekStartDays) {
    const startOfWeekYearDate = startOfWeekYear(date, { weekStartsOn: weekStartDay });
    console.log(`Start of week-numbering year for ${date.toDateString()} (week starts on ${weekStartDay}): ${startOfWeekYearDate.toDateString()}`);
  }
}

import startOfWeekYear from 'date-fns';

const dates = [
  new Date(2024, 0, 1), // January 1, 2024
  new Date(2024, 6, 2), // July 2, 2024
  new Date(2024, 11, 31), // December 31, 2024
  new Date(2025, 1, 1), // January 1, 2025
  new Date(2025, 6, 2), // July 2, 2025
  new Date(2025, 12, 31), // December 31, 2025
];

const firstWeekDates = [1, 2, 3, 4, 5, 6, 7]; // January 1st to 7th

for (const date of dates) {
  for (const firstWeekDate of firstWeekDates) {
    const startOfWeekYearDate = startOfWeekYear(date, { firstWeekContainsDate: firstWeekDate });
    console.log(`Start of week-numbering year for ${date.toDateString()} (first week contains ${firstWeekDate}th of January): ${startOfWeekYearDate.toDateString()}`);
  }
}

Manual Calculation

You can manually calculate the start of the week-numbering year by following these steps:

Identify the Week-Numbering Year
Use the getWeekYear function from "date-fns" or a similar method to determine the week-numbering year associated with the given date.
Set the Initial Date
Create a new date object using the given date but set the day, month, and year to their minimum values (e.g., January 1st of the same year).
Adjust the Date to the Week-Numbering Year
Set the full year of the initial date to the previously determined week-numbering year.
Determine the Day of the Week
Calculate the day of the week for the adjusted date.
Adjust the Date to the Week Start
Subtract the day of the week from the day of the month to ensure the desired first day of the week.
Return the Start Date
The resulting date represents the start of the week-numbering year for the given date.

Using Other Libraries

Some other JavaScript libraries, such as "Moment.js" or "Luxon," provide similar functionality to startOfWeekYear. You can explore these libraries and their specific implementations if they better suit your preferences or project requirements.

Custom Logic

If you have specific requirements or need more control over the calculation, you can write your own custom logic to determine the start of the week-numbering year. This approach allows for greater flexibility but may require more effort and careful implementation.

Considerations

For production-grade code, it's generally recommended to use well-tested and maintained libraries like "date-fns" or "Moment.js" for date and time manipulation tasks.
If you're manually calculating or writing custom logic, ensure that you handle edge cases and account for different week start conventions and first week dates.
When choosing an alternative method, consider factors such as performance, ease of use, and compatibility with your existing codebase.