JSON does not support serialization of associative (key-value pair) arrays of the kind
var a = new Array(); a['a'] = 'a'; a['b'] = 'b';
That means Socket.IO and other code that relies on JSON serialization will not work.
It is also not wise to use large numeric values as the array index, for instance
var a = new Array(); var id = '1000'; a[id] = 'foo';
That will result in an array with 1001 items, where the first 1000 items store nothing. This array when serialized will result in a really long JSON string.
The solution is to use a sequential counter, or use the push method to add an object into the array. Of course, finding an object in a large array will be slow. You’ll have to scan array items sequentially till you find the desired object.