this post was submitted on 06 Nov 2023
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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 yogthos@lemmy.ml to c/programmerhumor@lemmy.ml
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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@sh.itjust.works 1 points 1 year 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.