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Comment by JeanMeche | Hacker News Reader

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JeanMeche

You might want to play with https://jsdate.wtf/

One can't fathom how weird JS Date can be.

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Guessed 2 of the first 3 questions.

Got to question 4 and gave up:

    new Date("not a date")
    1) Invalid Date
    2) undefined
    3) Throws an error
    4) null

There's literally no way of guessing this crap. It's all random.

the new Date() constructor is an amalgamation of like 5 different specs, and unless the input matches one of them, which one kicks in is up to the implementer's choice

The choice here is really surprising. I was half-expecting NaN, that you omitted.

Is there any other instance of the standard JS library returning an error object instead of throwing one? I can't think of any.

I think NaN itself is a bit of an error object, especially in how it's passed through subsequent math functions, which is a different choice than throwing up.

But besides that I think you're right, Invalid Date is pretty weird and I somehow never ran into it.

One consequence is you can still call Date methods on the invalid date object and then you get NaN from the numeric results.

The fun trick is that Invalid Date is still a Date:

    > let invalid = new Date('not a date')
    > invalid
    Invalid Date
    > invalid instanceof Date
    true

You were half-correct on expecting NaN, it's the low level storage of Invalid Date:

    > invalid.getTime()
    NaN

Invalid Date is just a Date with the "Unix epoch timestamp" of NaN. It also follows NaN comparison logic:

    > invalid === new Date(NaN)
    false

It's an interesting curio directly related to NaN.

> Invalid Date is just a Date with the "Unix epoch timestamp" of NaN. It also follows NaN comparison logic: > > > invalid === new Date(NaN) > false

This is just because a JS Date is an object and have nothing to do with the inner representation.

    > new Date(0) === new Date(0)
    false

I had no idea we even had an `Invalid Date` object, that's legitimately insane. Some other fun ones:

    new Date(Math.E)
    new Date(-1)

are both valid dates lol.

I think you got it backwards. The only way is guessing, it's all random.

It's the internal inconsistencies that get me. Like, OK, I understand that there might be some quirks, maybe due to some weird backwards compatibility or technical limitation, but there are multiple incompatible quirks _inside_ this single interface! It's terrible, and things like this are a huge part of the reason JS was long considered (and sometimes still is) a Not So Good language.

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