Are Javascript arrays sparse?

Question

That is, if I use the current time as an index into the array:

array[Date.getTime()] = value;

will the interpreter instantiate all the elements from 0 to now? Do different browsers do it differently?

I remember there used to be a bug in the AIX kernel, which would create pseudo-ttys on request, but if you did, say, "echo > /dev/pty10000000000" it would create /dev/pty0, /dev/pty1, .... and then fall over dead. It was fun at trade shows, but I don't want this to happen to my customers.

Answer 1

Yes, they are. They are actually hash tables internally, so you can use not only large integers but also strings, floats, or other objects. All keys get converted to strings via toString() before being added to the hash. You can confirm this with some test code:

<script>
  var array = [];
  array[0] = "zero";
  array[new Date().getTime()] = "now";
  array[3.14] = "pi";

  for (var i in array) {
      alert("array["+i+"] = " + array[i] + ", typeof("+i+") == " + typeof(i));
  }
</script>

Displays:

array[0] = zero, typeof(0) == string
array[1254503972355] = now, typeof(1254503972355) == string
array[3.14] = pi, typeof(3.14) == string

Notice how I used for...in syntax, which only gives you the indices that are actually defined. If you use the more common for (var i = 0; i < array.length; ++i) style of iteration then you will obviously have problems with non-standard array indices.