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:

Answer 1

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.