Why does the `map` method apparently not work on arrays created via `new Array(count)`?

arrays, javascript, map-function

Solution

It appears that the first example

x = new Array(3);

Creates an array with a length of 3 but without any elements, so the indices [0], [1] and [2] is not created.

And the second creates an array with the 3 undefined objects, in this case the indices/properties them self are created but the objects they refer to are undefined.

y = [undefined, undefined, undefined]
// The following is not equivalent to the above, it's the same as new Array(3)
y = [,,,];

As map runs on the list of indices/properties, not on the set length, so if no indices/properties is created, it will not run.

Problem

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] ``` What's going on here? Is this a bug, or am I misunderstanding how to use `new Array(3)`?

Original source