Are Arrays really "sparsed" in JavaScript by default?

How to delete single array element with no index shift?

What is the difference between:

var a = new Array(10);
var b = new Array(2);

Do these Arrays occupy different space in memory? Can I turn Array(2) into Array(10) and back?

UPDATE 1

The following code

<body>
<script type="text/javascript">

    var a = new Array(10);
    var b = new Array(2);

    document.write('a[5] is ' + a[5] + '<br/>');
    document.write('a[15] is ' + a[15] + '<br/>');
    document.write('a.length is ' + a.length + '<br/>');
    document.write('b[5] is ' + b[5] + '<br/>');
    document.write('b[15] is ' + b[15] + '<br/>');
    document.write('b.length is ' + b.length + '<br/>');

    b[9]=12;
    delete b[9];

    document.write('After resize...<br/>');
    document.write('b[5] is ' + b[5] + '<br/>');
    document.write('b[15] is ' + b[15] + '<br/>');
    document.write('b.length is ' + b.length + '<br/>');

</script>
</body>

will return

a[5] is undefined
a[15] is undefined
a.length is 10
b[5] is undefined
b[15] is undefined
b.length is 2
After resize...
b[5] is undefined
b[15] is undefined
b.length is 10

So, I made b have the same properties as a. Can I do this without assigning fake value to b[9]? Can I do revers, i.e. having a.length being 2?

Solution

Yes, arrays are sparse by default. Their properties aren't even defined. You can test it with this code:
```
var arr = new Array(10);
for(var x in arr) {
    alert(x); // Never alerts a number
}
```
To delete a single element, use delete:
```
delete arr[5]; // Deletes element 6.
```
Memory is likely irrelevant, and different engines will handle it differently. I don't think that can be answered accurately. As for changing dimensions, JavaScript arrays don't have fixed dimensions. This is perfectly legal:
```
var arr = new Array(2);
arr[5] = "Hello!";
```

You can read more about JavaScript in general on http://developer.mozilla.com/. Arrays are just objects with some extra special properties. It doesn't really have indexes so much as keys.