How to populate a blank form field with onSubmit

New to JavaScript so please forgive my ignorance.

I have a form where when the customer submits the form it checks a specific field and if it's left blank I need it to populate it with a specific code. Any ideas how I can do this?

<form m开发者_开发技巧ethod="post" action="index.php" name="checkout">

<table width="100%" border="0" cellpadding="5" cellspacing="10">
 <tr>
  <td align="right" class="normaltext" width="40%">
   <b>Catalog Code:</b>
  </td>
  <td align="left">
   <input type="text" name="cat_code" value="{$valid.cat_code}" size="25" class="formtext" />
  </td>
 </tr>
</table>

Then the submit button:

<input type="submit" name="submit" value="Continue &gt;" class="addtocart_btn btnstyle1" />

I guess you can use jQuery to do something like this:

<form id="checkout" name="checkout" method="post" action="index.php">
    <input id="input1" type="text" value="" />
    <input id="input2" type="text" value="" />
    ...
    <input id="inputn" type="text" value="" />
</form>

<script type="text/javascript">
    $("checkout").submit({
        if ($("#input1").val() == "") {
            $("#input1").val() = "Your desired value for input1";
        } 
        // put more if condition to populate the blank fields as needed
    });
<script>

Add an onsubmit handler to your form:

<script type="text/javascript">
    window.onload = function()
    {
        document.forms.checkout.onsubmit = checkBlankField;
    }
    function checkBlankField()
    {
        var field = document.forms.checkout.cat_code;
        if (!field.value)
        {
            field.value = "some value";
        }
    }
</script>