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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] 3 points 1 year ago (2 children)

What would you expect "-1 month" to do for a date like 31st of March? Would the result be the same as for "-1 month" on 29th of March?

If you go back 2 months so the 31st is existing again - should that mean that the result of using -1 month twice should be different to using -2 months?

I think it's just a stupid way to implement something like this as "month" isn't a defined size so defining it with a fixed value and documenting it properly is a decent solution but noone should use that kind of function in the first place

[–] [email protected] 4 points 1 year ago* (last edited 1 year ago)

It is a stupid way to implement it, but the called function is named setMonth()! The minus one is performed externally, so if you set February you expect February, validation should adjust the other fields...

[–] [email protected] 3 points 1 year ago

This is literally how every sane API works in languages built by adults. For example, here's what happens in Java:

java.time.LocalDate.of(2023, 3, 31)
> #object[java.time.LocalDate 0x2bc77260 "2023-03-31"]
java.time.LocalDate.of(2023, 3, 31).minusMonths(1)
> #object[java.time.LocalDate 0xac0dc15 "2023-02-28"]
java.time.LocalDate.of(2023, 3, 31).minusMonths(2)
> #object[java.time.LocalDate 0x44b9305f "2023-01-31"]

I have no idea where people get this notion that a month isn't a defined size. Do people just not understand the concept of a month?