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

    > invalid.getTime()
    NaN

    > invalid === new Date(NaN)
    false