Getting Data From An Arrayed Form Input?

I can get a value in a form input field with Javascript by doing this with an appropriate Javascript event and associated functions;

myval = parseInt(document.form_name.input_name);

or

myval = parseInt(getElementById(input_ID));

or put them in an array by

var val = new Array(2);
val[0] = parseInt(document.form_name.input_name);
val[1] = parseInt(document.form_name.input_name);

But how do I do this if the input name itself is a array like name= 'input_name[]'

or similarly if the input ID is an array i.e. ID= 'inputID[]'

I have tried a number if expressions but so far no luck.

Firebug will show the input ID like inputID0 or inputID1, but that doesn't seem to work for identifying the name or ID either.