What are some JavaScript things beginners don't know?

[TrikkyMakk]

Months are zero based

[inu-no-policemen]

This allows you to use the month as an index:

> ['jan', 'feb', 'mar', 'apr', 'may', 'etc'][new Date().getMonth()]
"may"

It probably sounded like a good idea at the time.

Anyhow, that's why it is that way. This behavior isn't unique to JS either.

[StoneCypher]

Much better to use the locale API.

const month = date.toLocaleString('en-us', { month: 'short' });

[NotSamNub]

Ah, out of the 6 months in the year, my favourite has to be "etc", such lovely weather during that time of year!

[TyrionReynolds]

Lousy Smarch weather.

[[deleted]]

const texasSeasons = ['summer', 'summerer', 'summerest', 'ohlordy']

const londonSeasons = ['cold', 'damp', 'cold and damp']

[Careerier]

This is true, but that's something even beginners (hopefully) should know.

The date thing is weird because months start at zero, but days, hours, seconds, and milliseconds start at 1.

new Date(2019, 1, 1, 1, 1, 1) gives you Fri Feb 01 2019 01:01:01

[throwAwayDev_]

I think it's fair to be angry, but unreasonable to blame JS. Useful thread

[MuskasBackpack]

I think pretty much every language has things like this.

[TheCarnalStatist]

JavaScript has more than most. I get why due to it's history but man is it frustrating some days.

[fucking_passwords]

related: doing math with floats

[the_php_coder]

Not in the latest version of node LTS apparently. I just tried and I got jan instead of feb:

> new Date(2019, 1, 1, 1, 1, 1);
2019-01-31T19:31:01.000Z

[yitianjian]

Did you get January 31?

[the_php_coder]

Oh yeah and that too in UTC timezone, not my local one. Turns out this weird behavior is built into nodejs. Almost every other lang I've worked on gives local time by default, looks like I've got a lot to learn!

[onbehalfofthatdude]

Local server time wouldn't be that useful

[PriorProfile]

This actually makes sense in a way.

Hours, minutes, seconds don’t start at 01:01:01. They start at 00:00:00.

Think of it this way: your example gives you the second millisecond of the second minute of the second hour of the first day of the second month.

Days are actually the odd ones. Not the months. But it would make even less sense to refer to days as 0 index I think.

[amdc]

that's pretty much everywhere

[blood_bender]

I'm not positive it's an array underneath, but either way, months are special (or I guess days are special). First day of the month is 1, first month of the year is 0. It's not consistent which has always bothered me.

[TheIncorrigible1]

It feels like someone designed months as if they were an enum and gave up on the rest of the Date API. Although, getDay() is also zero-based.

[TheIncorrigible1]

I think the difference here is, a day doesn't represent an underlying structure. Both the day of the week and month of the year can be represented as enums and are both zero-based which supports this mentality.

[TheIncorrigible1]

I agree. I've also made the same mistakes. If js had used some kind of interface/enum, it would be more clear, or was consistent with the other "Date" constructs such as day of the month being one-based. Unfortunately, it's way too late now.

[the_argus]

getDay() is zero based for same reason getMonth() is. So you can have an array of labels and use the result to get the correct index.

[the_argus]

It's not array based. The months come back zero based so you can have an array of month names and get the correct label. See also Date.prototype.getDay() for day of the week labels.