Following up on this thread:

Lots of Null Values in an array mean any harm?

I did this with node.js:

arr=[]
arr[1000]=1
arr[1000000000]=2
arr.sort()

And I got

FATAL ERROR: JS Allocation failed - process out of memory

So that leaves me with the question (I couldn't find it on Yahoogle) how much Memory is actually allocated for a null entry in an array in node. I do not plan to use 1000000000 entries, not even close, but maybe it's still not worth allocating the memory...

Who knows how I can check?

So in summary, node.js does not allocate memory for undefined values in an array. The crash I experienced must have been a glitch as no one else could reproduce it and installing the latest node.js version eliminated the problem for me as well.