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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 10 months 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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[–] ricecake 1 points 10 months ago (1 children)

Have you never done Datetime math before?

You didn't run new Date(2019, 02, 31), you asked it for one month from January 31st.
One month after any given day of the month is, by most people's intuition, a valid thing to ask for.

Your solution of making the API throw an exception for every 31st of the year is vastly less intuitive to me than "a month is 31 days in January, and 28 or 29 in February, so adding a month does different things in different months", because at least for those is can also query how many days are in the month.

If I say to add four days, will it throw an exception if it's the 31st? No month has 35 days, so I should get an exception, right?
Or is it just this weird caveat around months? Does it apply to time?

[+] [email protected] -7 points 10 months ago* (last edited 10 months ago) (1 children)

One month after any given day of the month is, by most people’s intuition, a valid thing to ask for.

Nobody actually thinks this way, when you ask somebody what's the next month they don't go, oh today is the x day of the month and I'm going to add days to the current date to see what's the next month. That's not how vast majority of people think about this intuitively. Clearly you and people who designed Js Date API do though.

“a month is 31 days in January, and 28 or 29 in February, so adding a month does different things in different months”

That's how months work. Each month has a different number of days.

If I say to add four days, will it throw an exception if it’s the 31st? No month has 35 days, so I should get an exception, right?

Yeah, it's how months work.

In fact, this isn't a hypothetical argument this is how date APIs work in sane languages like Java. For example:

java.time.LocalDate.of(2023, 2, 31)
> java.time.DateTimeException: Invalid date 'FEBRUARY 31'

java.time.LocalDate.of(2023, 2, 3)
> #object[java.time.LocalDate 0x2bc77260 "2023-02-03"]

The fact that you think what Js API dies is preferable to this is frankly surreal to me.

[–] ricecake 1 points 10 months ago (1 children)

So, you're talking about throwing exceptions if adding a month yields an invalid date if done without nuance, but then you're showing an example of just trying to instantiate an invalid date in java.

I believe java has an addMonths method, that in the situation we're talking about doesn't throw an exception, but rather limits the output to the 28th/ 29th/30th.

Which illustrates my point: intuitive Datetime math involves choosing how you handle edge cases that are routine and not exceptional, like "a month from January 31st".

The rest of your comment arguing about how people expect months to work just makes me feel like you've never actually talked to a business stakeholder about requirements.

If I say to add four days, will it throw an exception if it’s the 31st? No month has 35 days, so I should get an exception, right?

Yeah, it's how months work.

That's just insane. An API that doesn't increment the month when you pass the end while adding days is just broken.