HTML form input tag name element array with JavaScript

This is a two part question. Someone answered a similar question the other day (which also contained info about this type of array in PHP), but I cannot find it.

1.) First off, what is the correct terminology for an array created on the end of the name element of an input tag in a form?

<form>

    <input name="p_id[]" value="0"/>
    <input name="p_id[]" value="1"/>
    <input name="p_id[]" value="2"/>

</form>

2.) How do I get the information from that array with JavaScript? Specifically, I am right now just wanting to count the elements of the array. Here is what I did but it isn't working.

function form_check(){
    for(var i = 0; i < count(document.form.p_id[]); i++){  //Error on this line

        if (document.form.p_name[i].value == ''){
            window.alert('Name Message');
            document.form.p_name[i].focus();
            break;
        }

        else{
            if (document.form.p_price[i].value == ''){
                window.alert('Price Message');
                document.form.p_price[i].focus();
                break;
            }

            else{
                update_confirmation();
            }
        }
    }
}