Working with checkbox in PHP and JS

I am creating a form with both radio buttons and checkboxe开发者_JS百科s. I want to validate the form with both javascript onsubmit, and php. For php to work, I have to write the checkboxes and radiobuttons with [] in the name (e.g. ) I searched the other topics and saw that I can put a default checked value for radiobuttons to avoid the probably of people not picking a value. That's fine. However, I also need to make sure people check no more than 3 check boxes. This cannot be done with

var count = 0;
for (x=0; x<document.fic_rec.rec_genre.length; x++){
    if (document.fic_rec.rec_genres[x].checked){
        count++;
    }
}

if (count==0){
    rec_error += "at least 1 genre; ";
}
else if (count>3){
    rec_error += "no more than 3 genres; ";
}   

I get an error saying "Cannot read property 'length' of undefined My HTML code looks like this:

<ul class="sub_ul">
<li class="sub">
<input type="checkbox" name="rec_genres[]" id="action"value="action" />
<label for="action">Action/Adventure</label></span><li class="sub"><u>Alternative reality</u></li>
<li class="sub_sub"><input type="checkbox" name="rec_genres[]" id=au_history     value="au_history" />
<label for="au_history">AU History</label></li><li class="sub_sub"><input type="checkbox" name="rec_genres[]" id=au_universe value="au_universe" />
<label for="au_universe">AU Universe</label></li><li class="sub_sub"><input type="checkbox" name="rec_genres[]" id=au_other     value="au_other" />

etc etc

Help! Thanks!!

Use e.g. the elements-collection to access those elements:

document.fic_rec.elements['rec_genres[]']

Please note: This will return a nodeList if there are more than 1 element found.
If there is only 1 element with the given name, it will return the element itself and not a nodeList.

You may also use querySelectorAll() (not supported by IE<8)

document.querySelectorAll('form[name="fic_rec"] input[name="rec_genres[]"]')

...this will always return a nodeList, no matter how many elements are found.

Answer to the comment:

var radio_count = 0;         
for (var i=0;i<document.fic_rec.elements['rec_pub_length[]'].length;i++) 
{ 
  if (document.fic_rec.elements['rec_pub_length[]'][i].checked) 
  { 
    radio_count = 1; 
    break;
  } 
} 
if (radio_count != 1) { rec_error += "story length; "; }