Adding a bulk of items in the middle of a sparse array
You don't need to worry about sparse arrays in JavaScript and there is no need to instantiate with the Array constructor. If you insert into a random index then the rest of the array is just padded with undefined references:

var items = [];
items[5] = 'foo';
console.log(items); // [5: "foo"]
items.toString();   // ",,,,,foo"


To add new items into the middle of an array you can use the splice function:

items.splice(index, 0, bulk);


The index is where to start adding items, 0 is how many items to remove after that point, and bulk is the new items to add at that point. Splice acts on the array directly, and returns any items that were removed.