Webmaster Forum / HTML, CSS, Scripts / JavaScript / March 2005

But you don't get silent failure. The date is wrapped so that it
becomes valid. I think that's pretty useful, so what would be more
useful in your opinion?

Mike

 As Mike said, there is no failure.  The expected behaviour is that if
 no date is specified when setMonth() is called, the value returned
 from getUTCDate() is used.

 If the date is 2005-mar-31 and you use setMonth(1) to create a date
 of 2005-feb-31, the expected behaviour is that parameters are updated
 in accordance with the ECMA 262 spec and 2005-03-03 is returned.

 E.g.

  var n = new Date();
  n.setMonth(n.getMonth + 15)

  will effectively add 1 to the year and 3 to the months. And given
  that for today (2005-03-31) that will create a date of 31 June, the
  returned date is 01 July 2005

 So while the result you get it not what you expect, it is not
 'failure' and is precisely according to the specification.

 You need to determine your requirements - maybe you are trying to
 find the date of the last day of the month for a month x months
 previous or hence?  In that case, the most efficient method does not
 require a date object at all, just numbers for month and year.

Sorry, that's not how Date works. You will have to create your own
method that checks whether the setting succeeded:

function setMonth(date, month) {
 date.setMonth(month);
 if (date.getMonth() != month) {
   throw "invalid date for month";
   // notice: date has already been updated.
 }
}

The change to the date object is not a silent failure. It is deliberatly
treating the 31th of Februaray this year as an alternative way to write
the 3rd of March.

/L