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:
It appears that the first example
x = new Array(3);
Creates an array with undefined pointers.
And the second creates an array with pointers to 3 undefined objects, in this case the pointers them self are NOT undefined, only the objects they point to.
y = [undefined, undefined, undefined]
// The following is not equivalent to the above, it's the same as new Array(3)
y = [,,,];
As map is run in the context of the objects in the array I believe the first map fails to run the function at all while the second manages to run.
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));
Update
Another solution could be:
let arr = Array.apply(null, Array(10)).map((val, idx) => idx);
console.log(Array.apply(null, Array(10)).map((val, idx) => idx));