How are empty cells stored in a sparse array in JavaScript?

Question

I have a situation where I may be setting an array cell of a high index without setting any of the cells before it.

>>> var arr = [];
undefined
>>> arr[5] = 'value';
"filled"
>>> arr
[undefined, undefined, undefined, undefined, undefined, "filled"]

How is such an array stored in memory? Is there space allocated for each undefined value?

In my actual project, I may be using very large indices. For example, I may set cells 500-800 and 900-1000. I can't use a hash because I need to loop through these non-empty cells and be aware of their index. I want to know if fragmenting the array like this will use up a ton of memory for the empty cells.

Answer 1

I can't use a hash because I need to loop through these non-empty cells and be aware of their index.

What's wrong with the for (x in ...) language construct?

EDITED to accomodate vol7ron 's comment:

var x = {2: "foo", 999: "bar"};

for ( var n in x ) {
    if ( x.hasOwnProperty(n) ) {
        console.log(n);
        console.log(x[n]);
    }
}  

Answer 2

Strapping onto aefxx's answer, you can still iterate:

var obj = {500: "foo",  10923: "bar"};
var max = 0;

for (var key in obj)
   max=key>max?key:(max||key);         // get max key

for (var i=0; i<=max; i++)
   console.log(obj[i]);                // output even the undefined

---

As Phrogz commented, it doesn't allocate for undeclared elements of the array. I'm not certain if that's the case if you explicitly set the element value to undefined (eg arr[somenum] = undefined; )

Answer 3

You should probably simply store the max index in a variable and the access your map like this :

for (var i=0; i<=maxIndex; i++) {
 console.log(myMap[i]);
}

Then you'll have the (relative) compacity of the map and the ability to loop through unsetted indexes.