javascript - Curious behavior of String()

Question

I was just messing around with some code today, and I noticed that when I run String(null) or String(undefined), I get null and undefined respectively. But, when I checked the value I got for String([null, undefined]), I found it gave me "," as compared to the expected Result null,undefined. Does anyone have any idea on why this is happening? Since I didn't expect such behavior from String constructor on arrays, because as far as I have Noticed, the constructor simply put the values of every single one of the elements separated by commas.

Answer 1

String(x) calls x.toString().

null and undefined values are represented by empty strings by Arrays' toString() since it calls Array.prototype.join() ("If element is undefined or null, let next be the empty String"):

> [null, undefined].toString()
","
> [null, null, null, null].toString()
",,,"