Confused by behavior of `map` on arrays created using `new` [duplicate]

Question

I am confused by the results of mapping over an array created with new:

function returnsFourteen() {
    return 14;
}

var a = new Array(4);
> [undefined x 4] in Chrome, [, , , ,] in Firefox
a.map(returnsFourteen);
> [undefined x 4] in Chrome, [, , , ,] in Firefox

var b = [undefined, undefined, undefined, undefined];
> [undefined, undefined, undefined, undefined]
b.map(returnsFourteen);
> [14, 14, 14, 14]

I expected a.map(returnsFourteen) to return [14, 14, 14, 14] (the same as b.map(returnsFourteen), because according to the MDN page on arrays:

If the only argument passed to the Array constructor is an integer between 0 and 2**32-1 (inclusive), a new JavaScript array is created with that number of elements.

I interpret that to mean that a should have 4 elements.

What am I missing here?

Answer 1

When you create an array like so:

var arr1 = new Array( 4 );

you get an array that has a length of 4, but that has no elements. That's why map doesn't tranform the array - the array has no elements to be transformed.

On the other hand, if you do:

var arr2 = [ undefined, undefined, undefined, undefined ];

you get and array that also has a length of 4, but that does have 4 elements.

Notice the difference between having no elements, and having elements which values are undefined. Unfortunately, the property accessor expression will evaluate to the undefined value in both cases, so:

arr1[0] // undefined
arr2[0] // undefined

However, there is a way to differentiate these two arrays:

'0' in arr1 // false
'0' in arr2 // true

Answer 2

var a = new Array(4);

This defines a new array object with an explicit length of 4, but no elements.

var b = [undefined, undefined, undefined, undefined];

This defines a new array object with an implicit length of 4, with 4 elements, each with the value undefined.

From the docs:

callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes which have been deleted or which have never been assigned values.

For array a, there are no elements that have been assigned values, so it does nothing.

For array b, there are four elements that have been assigned values (yes, undefined is a value), so it maps all four elements to the number 14.