2y ago

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.

    
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.

47 comments

You want to create the date "31st February", but it's JavaScript that's cursed?
Write a less side-effecty function.

function getMonthName(monthNumber) { const date = new Date(2023, monthNumber - 1, 1); return date.toLocaleString([], { month: 'long' }); }
- 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.
  
  The rake has nothing to do with JS (which I agree is cursed, but for its own reasons, not this).
  You have called a function in a way that does not give a consistent value (Date()). Such functions are hardly the preserve of JavaScript. You've failed to adequately deal with the range of values produced, with code that tries to insist that the "31st February" can be a meaningful date in February. You should accept that this is your mistake and learn to (better) avoid side effects where possible.
  Also, the function isn't side effecty since it doesn't make implicit references outside its scope.
  Edit responding to your edit:
  Also, the function isn't side effecty since it doesn't make implicit references outside its scope.
  The Date() function's output varies according to something other than its input (and even the rest of your program). Using its output without accounting for that variation means that your function, as originally written, also gives inconsistent return values, varying according to something other than its input, because it does, in fact, reference something outside the function. If it did not, the results would only depend on the monthNumber argument, and would always be consistent. I don't know what you call that, but I view it as a side effect.
  As you have said, the rake is that months have different lengths, and you need to account for that. But that's not one of JavaScript's many issues.
The legacy Date object has many problems and this is one of them. Another infamous one is that it uses zero-based month numbers: January is the zeroth month and December the 11th month.
This will be fixed Any Day Now™️ when Temporal is released. This is a carefully designed library that supersedes Date and is currently waiting on some standards to be finalized.
date.setDate(1);
Problem solved.
- Sure, but the question is why anybody thought this would be a desirable behavior in the first place.
  
  It's because there's no right answer, and this way gets you the intuitive answer most often.
  A month isn't a proper unit of time. Adding a month to a date can't be done without specifying which month you're adding.
  You could argue that one month from January 31 is February 28, 29 (depending on the year), March 2, or 3.
  Should one month from the last day be the last day of the next month? That would mean that the 30th and the 31st of march are both the same duration from April 30th, and a month before April 30th could logically map to either one.
  So they chose the path that, for anything other than the 31st, and the 29th and 30th if it comes near February, works as you expect. "A month after 17 days from the first of January is 17 days after the first of february.”
  The other alternatives involve not allowing the addition and subtraction of irregular time intervals, but then you get frustrated that you can only deal with seconds, since those don't change in length.
  
  I love js. But the date object has always been a total pain. Moment.js is a good package to deal with it, but yeah, it's currently deprecated, but it would be nice if it or something like it became part of ECMAScript.
  I have no idea why it hasn't yet, except that it might be that js needs to work for everyone, not just the us. So time is not standard.
Oh, because if the month you chose has less than 31 days, it'll assume the 31st of September is the 1st of October? That's reasonable.
Somebody has not worked with dates for very long, to be so sure of themselves...
- I've worked with dates long enough in lots of different languages to recognize API decisions that are absurd.
The amount of people arguing that this is a fine behavior in this thread makes the whole thing even funnier.
- 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
  
  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...
  
  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?
- I would expect the month to increment by one and the day to be clamped to the valid days for the month.
  
  That's precisely what I'd expect as well, and what APIs in languages like Java do.

47 comments