Why do I get undefined values in my array?

Question

Why is the value of

(new Array(2)).map(function (x, i, a) { return i })

[undefined, undefined] instead of [0, 1]?

You can use `Array(2).join("|").split("|").map(function (x, i, a) { return i })` or any map implementation that doesn't check if an index actually exists, like `jQuery.map` — Esailija, Nov 09 '12 at 14:00
@epascarello: Gonna do it, I was just searching for the related/duplicate question on JS list comprehension. — Bergi, Nov 09 '12 at 14:06

score 1 · Accepted Answer · edited May 23 '17 at 12:29

1

new Array(2) generates a sparse array - with no values, but of length 2. It is equivalent to [,,].

Now, Array's .map() method is specified to leave out uninitialised/deleted indices, so you just get back another empty array.

edited May 23 '17 at 12:29

Community

answered Nov 09 '12 at 14:04

Bergi

score 0 · Answer 2 · answered Nov 09 '12 at 14:04

0

when you specify the size of an javascript array it fills it with undefined values

answered Nov 09 '12 at 14:04

VuesomeDev

No, the point is that is does not fill with values. – Bergi Nov 09 '12 at 14:06
and it will return undefined, thats what i wanted to say – VuesomeDev Nov 09 '12 at 14:09

2 Answers2