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  3. JavaScript — Get Number of Days in a Month

JavaScript — Get Number of Days in a Month

In some situations, you need the number of days within a month. For example, when displaying statistics for each day of a month or when calculating a value with the unit “per day”.

This tutorial shows you how to retrieve the number of days in a month using JavaScript.

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Retrieve the Number of Days in a Month With JavaScript

JavaScript dates don’t have a method to get the last day of a month. You need to manually retrieve it. A naive and valid approach could be to add days to a date until the month changes. Then you have the last day in a month representing the number of days.

Another option is to use a trick in JavaScript related to dates. This approach is less verbose but needs more understanding of “why” this is working. The solution is to use day 0 of the “next” month which JavaScript translates to the last day of the current month.

Here’s a sample utility function daysInMonth retrieving the number of days within a month for a given date:

/**
 * Returns the number of days within the month of the given `date`.
 *
 * @returns {Number}
 */
export function daysInMonth (date) {  
  if (! date instanceof Date) {
    throw new Error('The [date] argument passed to "daysInMonth" must be a Date instance.')
  }

  const year = date.getFullYear()
  const month = date.getMonth() + 1

  /**
   * The trick here is the "next" month in combination with day "zero". Months
   * in JS dates are 0-based, we’re adding plus 1 to use the "next" month.
   * Day "zero" of JS date will use the last day of the previous month.
   */

  return new Date(year, month, 0).getDate()
}

Notice: the months in a JavaScript date instance are zero-based. That means the date.getMonth() function returns 0 for January, 1 for February, and so on. Adding a month results in the “next“ month, and using day “zero” lets JavaScript normalize the date to the last day of the previous month.

Here’s how you can use the daysInMonth function:

daysInMonth(new Date('2024-12-25'))  
// 31

daysInMonth(new Date('2024-02-01'))  
// 29

daysInMonth(new Date('2025-02-10'))  
// 28

daysInMonth(new Date('2024-13-25'))  
// NaN

Notice again: the first example from above using the date 2024-12-25 combined with the calculation of the "next" month seems to exceed the months of the year 2024. And that’s true, JavaScript proceeds with the next year 2025. And because we’re using day 0 in the calculation, JavaScript goes back to the last day of the previous year.

That’s it!

Marcus Pöhls

Marcus is a fullstack JS developer. He’s passionate about the hapi framework for Node.js and loves to build web apps and APIs. Creator of Futureflix and the “learn hapi” learning path.

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