Why does new Date(undefined) create an invalid date, but new Date(null) does not?

Question

I noticed something odd today with Javascript:

console.log(new Date(null)); // 1970-01-01T00:00:00.000Z
console.log(new Date(undefined)); // Invalid Date

Why is this the case? I know null and undefined are not the same, but in this context I would expect the same result.

Answer 1

If new Date gets called with a single primitive argument that is not a string, it will cast it to a number. And while null will coerce to 0, undefined will become NaN, and that's the internal value of the date you're getting back.

console.log(null + ":")
console.log(Number(null))
console.log(new Date(null).valueOf())
console.log(new Date(null).toString())
console.log(undefined + ":")
console.log(Number(undefined))
console.log(new Date(undefined).valueOf())
console.log(new Date(undefined).toString())