JavaScript "new Array(n)" and "Array.prototype.map" weirdness

Question

I've observed this in Firefox-3.5.7/Firebug-1.5.3 and Firefox-3.6.16/Firebug-1.6.2

When I fire up Firebug:

var x = new Array(3)
console.log(x) 
// [undefined, undefined, undefined]

var y = [undefined, undefined, undefined]
console.log(y) 
// [undefined, undefined, undefined]

console.log( x.constructor == y.constructor) // true

console.log( 
  x.map(function() { return 0; })
)
// [undefined, undefined, undefined]

console.log(
  y.map(function() { return 0; })
)
// [0, 0, 0]

Answer 1

I had a task that I only knew the length of the array and needed to transform the items. I wanted to do something like this:

let arr = new Array(10).map((val,idx) => idx);

To quickly create an array like this:

[0,1,2,3,4,5,6,7,8,9]

But it didn't work because: (see Jonathan Lonowski's answer a few answers above)

The solution could be to fill up the array items with any value (even with undefined) using Array.prototype.fill()

let arr = new Array(10).fill(undefined).map((val,idx) => idx);

console.log(new Array(10).fill(undefined).map((val, idx) => idx));