Does null occupy memory in javascript?

Question

I\'ve got the following situation:

var large = [a,b,c,d,e,f,g,h,i];
var small = [a2, b2, c2, null, null, null, null, null, null, i2];

where

Answer 1

I can't fully answer your question, because I'm not sure whether null takes up as much room as explicitly setting it to undefined. But I believe the standard way to do this is to use the Array constructor:

var large = [1,2,3,4,5,6,7];
var small = new Array(large.length);
small.length; // 7
small[0]; // undefined

I believe this uses less memory than explicitly setting values to undefined, though the effect is the same. I've used this for a sparse array with 250,000+ indices, and didn't experience any memory issues (using Google Chrome).

Edit: (after playing around and doing a little more research) Lots of folks don't like the new Array() constructor, for lots of reasons (see e.g. this discussion). But I think it has a couple of advantages for large, sparse arrays:

• It sets the length property properly. If you need the array length to match your large array, you can't do this otherwise without setting it explicitly (small.length = large.length) or putting something undefined at the end (small[large.length-1] = undefined).

• It doesn't set keys for empty indices, which leads me to believe it uses less memory. If you have an array a = [null,null,null] or a = [undefined,undefined,undefined], those arrays each have three indices, and if you use a.map or a.forEach your function will be called for each index. If you use a = new Array(3) and then call a.forEach, your function will only be called for the indices you've explicitly set to a value post-construction.

But there doesn't seem to be a big performance difference between:

var a = new Array(2000); 
a[2000] = 1;

and:

var = [];
a[2000] = 1;

which also gives you an array of length 2000 with only one defined index. So if you don't care whether the length of small matches the length of large, I'd just use var a = [] and have it resized dynamically as necessary - from the simple perspective of setting and getting values at specific indices, it's exactly the same.