Prototype question

[John Farrel]

I have this function:

 
function indexOfOption(options, value)
{
    for (var i=0;i<options.length;i++)
    {
        if (options[i].value == value)
            return i;
    }
    return -1;
}

which I want to turn into a protoype method on a select.options array.
I thought I could do this:

function indexOfOption(value)
{
    for (var i=0;i<this.length;i++)
    {
        if (this[i].value == value)
            return i;
    }
    return -1;
}

Array.prototype.indexOfOption = indexOfOption;

But that doesn't work. The method is not recognised.
What am I doing wrong?

[John Farrel]

So no-one has any ideas?