JavaScript Array weird initialization

Question

I've stumbled upon something strange.... Maybe someone can try explaining the difference between these two:

var a1=Array(2);
var a2=Array.apply(null,a1);
console.log(a1);
console.log(a2);
console.log(a1[0]===a2[0]);

notice the items are equal but the console log of the arrays looks different (on Chrome and NodeJS).

also other aspects of them seem to work differently:

a1.map(function(i){console.log("!");})
a2.map(function(i){console.log("!");})

notice that map itereares only for a2 not for a1.

this happens on NodeJS, Chrome and FF.

Without checking the documentation, I'd say that a1=Array(2) sets a1 to an array with length 2, but doesn't explictly assign any elements. Whereas a2=Array.apply(null,a1) is like saying a2=Array(undefined,undefined), i.e., it explicitly sets the value of two elements to undefined. .map() only iterates elements that have been explicitly set. — nnnnnn
– nnnnnn, Commented Sep 27, 2014 at 6:23
In other words, (I'm guessing that in this case) the key is that someArray[someIndex] will return undefined for two possible reasons: 1. because the element at someIndex was never assigned a value, or 2. because the element at someIndex was explicitly assigned undefined. And your a1[0]===a2[0] test is comparing reason 1 undefined with reason 2 undefined. — nnnnnn
– nnnnnn, Commented Sep 27, 2014 at 6:26

Hrishi · Accepted Answer · 2014-09-27 06:50:42Z

4

In JavaScript, this creates a sparse array:

var a = new Array(3) // a = [ , , ,]

if you try to iterate over a using map or forEach, JavaScript skips the holes. The invocation

var b = Array.apply(null, Array(3)) // b = [undefined, undefined, undefined]

(this is equivalent to invoking the array constructor with Array(undefined, undefined, undefined))

creates a dense array b. b is almost the same as a but now, you can iterate over the elements, in other words, since the array is now dense, map and foreach don't skip over elements.

edited Sep 27, 2014 at 6:50

answered Sep 27, 2014 at 6:45

Hrishi

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1 Comment

srfrnk Over a year ago

10x for the pointer.... looked into it further and found you can actually distinguish the two cases with: console.log(0 in a1,0 in a2);

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