CODEWITHSUNDEEP
javascriptdate
Devanshi Sukhija
Devanshi Sukhija

Reputation: 167

Why does JavaScript: new Date(year, month, 0).getDate() return the number of days in the month?

I know that this little javascript code,var whatever = new Date(year, month, 0).getDate(), returns the number of days in a particular month of a particular year. But what I don’t seem to understand is the logic behind it.

What exactly is that zero doing there after we mention the year and the month? Please Explain.

Upvotes: 7

Views: 6270

Answers (2)

yongjie Yuan
yongjie Yuan

Reputation: 11

If any parameter overflows its defined bounds, it "carries over". For example, if a monthIndex greater than 11 is passed in, those months will cause the year to increment; if a minutes greater than 59 is passed in, hours will increment accordingly, etc. Therefore, new Date(1990, 12, 1) will return January 1st, 1991; new Date(2020, 5, 19, 25, 65) will return 2:05 A.M. June 20th, 2020.

Similarly, if any parameter underflows, it "borrows" from the higher positions. For example, new Date(2020, 5, 0) will return May 31st, 2020.

MDN

Upvotes: 1

David Schwartz
David Schwartz

Reputation: 182819

When you give a parameter out of range, the next larger time increment is adjusted to make the time valid. So:

> new Date(2016,2,1)
2016-03-01T08:00:00.000Z

So if we specify (2016,2,1), we get 3/1. So if we specify (2016,2,0), we'll get a day before that, adjusting the month as necessary to get something valid, that is, the last day of the previous month.

> new Date(2016,2,0)
2016-02-29T08:00:00.000Z

Upvotes: 10

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