Hi! I found that new Array(3) is not produce [undefined, undefined, undefined]. It produced [ <3 empty items> ]. And we are cannot to interact with it by Array.prototype methods. We must to convert it to array with Array.from for example.

> new Array(10)
[ <10 empty items> ]
> Array.from(new Array(10))
[ undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined ]

You can create arrays like that: new Array(n), where n is quantity of elements. For example, new Array(3) will produce [undefined, undefined, undefined]. Besides, every array has join method, which creates a string from the array's elements and join's argument will play as separator, so that [1,2,3].join('anyStringYouLike') will be evaluated in '1anyStringYouLike2anyStringYouLike3'. And [undefined, undefined, undefined].join('String') will be evaluated in 'StringString'.

Hope this explanation will help to undestand this solution.

I believe Array is a class, and when passed a number like this, it returns an empty array of size n + 1 (empty meaning, each element is undefined).

Example:

a = Array(3)
[ , , ]
a.length
3
a[0]
undefined
a.join('hi')
'hihi'