Does A Form Element Know Its Own Index Number In The Forms[].elements[] Array

I have a form element that looks like this:

<input type="text" onchange=doSomething(this)>

and a function:

function doSomething(theField){
}

I know that within the function I can access properties of the field (e.g. theField.name and theField.value). But how do I access theField's index number in the form's elements[] array -- from the "this" reference that was passed to the function? I.E. if this field is elements[3], how can I get at that 3?