Comment by Kyro38

3 days ago

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

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

7 comments

Kyro38

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.

dvt 3 days ago
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.
winstonp 3 days ago

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
marcosdumay 3 days ago
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.
- jazzyjackson 3 days ago
  
  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.
- WorldMaker 2 days ago
  
  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.
  
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