Javascript form validation. Radio buttons and checkboxes. How to validate?

I'm having issues with Javascript form validation for radio and checkboxes.

I have a external .js script with all my function rules linked in the html head section.

My rules look like this for example:

    function IsValid4DigitZip( str ) {
    // Return immediately if an invalid value was passed in
    if (str+"" == "undefined" || str+"" == "null" || str+"" == "")  
        return false;

    var isValid = true;

    str += "";

    // Rules: zipstr must be 5 characters long, and can only contain numbers from
   // 0 through 9
   if (IsBlank(str) || (str.length != 4) || !IsInt(str, false))
        isValid = false;

   return isValid;
} // end IsValid4DigitZip

And I call the function the section like this:

    if (IsValid4DigitZip(document.orderbooks.querySelectorAll("[name=postcode]")[0].value)) {

  } else {
    alert("Invalid postcode: You entered 3 or less digits. Please enter a 4 digit postcode");
    return false;
  }

Can some one help write function rules for radio and checkboxes using my examples.

I can only use Javascript. No Jquery.

Thank you.