Array.map doesn't seem to work on uninitialized arrays

If you'd like to fill an array, you can use Array(5).fill() and the methods will then work as expected--see the alternate related answer from aasha7. Older pre-fill approaches include:

Array.apply(null, new Array(5)).map(function() { return 0; });
// [ 0, 0, 0, 0, 0 ]

After some reading one of the posts linked in the comments, I found this can also be written as

Array.apply(null, {length: 5}).map(function() { return 0; });

---

However, trying to use .map on undefined values will not work.

x = new Array(10);
x.map(function() { console.log("hello"); });

// so sad, no "hello"
// [ , , , , , , , , ,  ]

.map will skip over undefined values :(

I'd just like to point out that you can now do:

Array(2).fill().map(_ => 4);

This will return [4, 4].

That's how it's described by the ECMAScript Language specification

Here's the relevant part of Array.prototype.map, as described by (§15.4.4.19)

• ...
• 8. Repeat, while k < len
  • a) Let Pk be ToString(k).
  • b) Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
  • c) If kPresent is true, then
    • do the magic
• ...

Since there is no initilized member in your array, calling e.g new Array (1337).hasOwnProperty (42)evaluates to false, hence the condition in step 8.c is not met.

You can however use a little "hack" to do what you want.

Array.apply(null, { length: 5 }).map(Number.call, Number) //[0, 1, 2, 3, 4]

How this works has been thouroughly explained by @Zirak