Modifying Dates with `setDayOfYear`: A Guide to Day Helpers in date-fns

• setDayOfYear(date, dayOfYear)
  
  • date: The date object you want to modify.
  • dayOfYear: The new day of the year value (1-based index).

The function likely performs the following steps:

1. • It might check if the provided date is a valid date object.
   • It may also check if the dayOfYear is within the valid range (1 to 366, considering leap years).

2. Day of Year Calculation
   

   • It calculates the new date by setting the day of the year to the provided dayOfYear value.
   • This might involve internal calculations to handle leap years and ensure the date stays consistent (e.g., February 29th on a non-leap year might be adjusted to February 28th).

3. Date Object Creation
   

   • It creates a new date object with the modified day of the year.

4. Return Value
   

   • The function returns the new date object with the updated day of the year.

In summary, setDayOfYear provides a convenient way to change the day of the year in a date object while maintaining date validity.

Here are some additional points to consider:

• For more advanced manipulation of dates, date-fns offers other helper functions like setMonth, setHours, etc.
• The specific implementation details might vary depending on the version of date-fns you're using.

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Example 1: Setting a Specific Day of the Year

// Import the function
import { setDayOfYear } from 'date-fns';

// Create a date object (July 14th, 2024)
const date = new Date(2024, 6, 14);

// Set the date to the 100th day of the year (around mid-April in most years)
const newDate = setDayOfYear(date, 100);

console.log(newDate); // Output: Approximately 2024-04-10 (depending on leap year)

Example 2: Handling Leap Years

// Import the function
import { setDayOfYear } from 'date-fns';

// Create a date object (February 29th, 2024 - Leap Year)
const leapYearDate = new Date(2024, 1, 29);

// Set the date to February 29th in a non-leap year (will be adjusted)
const nonLeapYearNewDate = setDayOfYear(leapYearDate, 29);

console.log(nonLeapYearNewDate); // Output: 2024-02-28 (adjusted to the last valid day)

// Import functions
import { setDayOfYear, format } from 'date-fns';

// Create a date object (today's date)
const today = new Date();

// Set the date to 60 days from today (approximately)
const futureDate = setDayOfYear(today, today.getDay() + 60);

// Format the new date for easier reading
const formattedDate = format(futureDate, 'yyyy-MM-dd');

console.log(formattedDate); // Output: YYYY-MM-DD (formatted future date)

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Using Native JavaScript Date Object Methods

function setDayOfYear(date, dayOfYear) {
  // 1. Clone the date object to avoid mutation
  const newDate = new Date(date.getTime());

  // 2. Set the full year (considering leap years)
  newDate.setFullYear(new Date().getFullYear(), 0, 1); // Start from Jan 1st

  // 3. Add the desired day of the year (dayOfYear - 1) as days
  newDate.setDate(dayOfYear - 1);

  return newDate;
}

// Usage example
const date = new Date(2024, 6, 14);
const newDate = setDayOfYear(date, 100);

console.log(newDate); // Output: Approximately 2024-04-10 (depending on leap year)

Using Intl.DateTimeFormat (Limited Functionality)

The Intl.DateTimeFormat API provides basic formatting capabilities. While it can't directly modify the date object, you can format the date with the desired day of the year and then parse it back into a new date object. However, this approach might not handle leap years accurately.

• Intl.DateTimeFormat
  Limited to formatting and parsing, might not handle leap years reliably.
• Native Date Object
  It's more verbose and error-prone compared to setDayOfYear. Handling leap years requires extra logic.

Alternatives with Similar Libraries

If date-fns isn't ideal for your project, consider other popular date manipulation libraries that offer similar functionality:

• Luxon
  A modern library focused on simplicity and immutability, offering functions like setDayOfYear.
• Day.js
  A lightweight alternative to Moment.js with similar functionalities.
• Moment.js
  A mature library with a wide range of features, including setting the day of the year.