Question

What Javascript language rule leads to the following conversion weirdness?

new Date() - 2 => number
new Date() * 2 => number
new Date() / 2 => number

but

new Date() + 2 => string

I had thought that the + operator would use the valueOf() method of the Date object to convert it to a number. Like in the following example:

{valueOf: function() {return 1;}} + 2 => number

What is different in the case of a Date?

Answer 1

After studying the Javascript spec, there is indeed a special case for Date objects in the context of the addition operator:

http://ecma-international.org/ecma-262/5.1/#sec-11.6.1

Conversion of a Date object is handled differently than for other objects:

No hint is provided in the calls to ToPrimitive in steps 5 and 6. All
native ECMAScript objects except Date objects handle the absence of a
hint as if the hint Number were given; Date objects handle the absence
of a hint as if the hint String were given.

I think I understand the motivation, but as a language rule, this feels quite ugly.