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If you run this JavaScript function on the 31st of a month, the result will be a month off. The best part is that this is the intended behvaior. JavaScript is a cursed language. (lemmy.ml)

submitted 1 year ago by [email protected] to c/[email protected]

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function getMonthName(monthNumber) {
  const date = new Date();
  date.setMonth(monthNumber - 1);

  return date.toLocaleString([], { month: 'long' });
}

The current day of month will have an impact on the behavior of this method. Conceptually it will add the number of days given by the current day of the month to the 1st day of the new month specified as the parameter, to return the new date. For example, if the current value is 31st January 2016, calling setMonth with a value of 1 will return 2nd March 2016. This is because in 2016 February had 29 days.

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[–] [email protected] 0 points 1 year ago* (last edited 1 year ago)

The point is that this scenario exists in Js in the first place. It's a completely unnecessary rake left around for people to step on. Also, the function isn't side effecty since it doesn't make implicit references outside its scope. The fact that the date is mutable is an internal concern there. You could just as easily do

function getMonthName(monthNumber) {
  const date = new Date();
  date.setDate(1);  
  date.setMonth(monthNumber - 1);

  return date.toLocaleString([], { month: 'long' });
}

The problem here isn't with side effects, but with having to know that you want to set your date to first day to get the next month reliably.