Matching form entries with javascript?

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?

From stackoverflow

• 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