Unobtrusive - adding a method, how to find a dependent property

Unobtrusive - adding a method, how to find a dependent property

I am writing an MVC app and I have created a custom validator, now I am trying to write the client side.  The intent is "if the dependant property is true (it's a radio") then this value must not be 0".

I am currently unable to get the value of radioValue, so the program fails at that point.  What am I doing wrong?

$.validator.addMethod("requiredifyes", function (val, elem, param) {
    var dependantProperty = param;
    var radioValue = document.getElementByName(dependantProperty).value;
    var counterValue = val;

    if (dpElement = true) {
        if (counterValue = 0) {
            return false;
        }
    }
    return true;
});

$.validator.unobtrusive.adapters.addSingleVal("requiredifyes", "othervalue", "requiredifyes");