Matching form entries with javascript?

Question

Hello,

I have a form with markup like this....

Which voucher did you cut out

<div class="answer item1">
  <input type="checkbox" name="downloaded_vouchers[answer1]" id="answer1" value="1"/>
   <label for="answer1">Answer 1</label>
</div>
<div class="answer item1">
   <input type="checkbox" name="downloaded_vouchers[answer1]" id="answer1" value="1"/>
   <label for="answer1">Answer 1</label>
</div>

Which voucher did you use

 <div class="answer item1">
  <input type="checkbox" name="used_vouchers[answer1]" id="usedanswer1" value="1"/>
   <label for="answer1">Answer 1</label>
</div>
<div class="answer item1">
   <input type="checkbox" name="used_vouchers[answer1]" id="usedanswer1" value="1"/>
   <label for="answer1">Answer 1</label>
</div>

I need to be able to somehow, on submition of the form check that the voucher cut matches the one that was used, there is some validation server side but I need to do some client side as well and have no idea where to start, I need to use straight javascript no jquery or similar, can anyone help?

Answer 1

something like this....

   <script type="text/javascript">
    function validate( )
    {
        // use document.getElementById to get the form item
        var item = document.getElementById("yourElement");
        if ( somecondition == true )
        {
            alert("good to go");
            // allow the form to post
            return true;
        }
        else
        {
            alert("I don't think so");
            // return false so the form will not post
            return false;
        }
    }
    </script>

    <form onsubmit="Validate();" >
    </form>

returning true from the onsubmit method will send the form through, returning false will not