Disable field based on value of option group

[bravo14]

I want to be able to disable a payment option if a user selects Delivery as an option.

 

the function I have is

 

function disableCOD(){ 
if (document.getElementById('optDelivery').value == 'optDel') { 
document.getElementById('optPaypal').checked = 'true'; 
document.getElementByName('optPayment').disabled='true'; 
} 
else{ 
document.getElementByName('optPayment').disabled='false'; 
} 
} 

 

The form details are

 

 

<table width="550" border="0" align="center" cellpadding="5" cellspacing="1" class="entryTable">
      <tr>
        <td width="150" class="entryTableHeader">Delivery Method </td>
        <td class="content">
        <input name="optDelivery" type="radio" id="optSwindon" value="Collect from Birmingham" onclick="disableCOD()"/>
        <label for="optBirmingham" style="cursor:pointer">Collect from Birmingham</label>
        <input name="optDelivery" type="radio" id="optSomerset" value="Collect from Somerset" onclick="disableCOD()"/>
        <label for="optSomerset" style="cursor:pointer">Collect from Somerset</label>
        <input name="optDelivery" type="radio" value="Delivery" id="optDel" onclick="disableCOD()"/>
        <label for="optDel" style="cursor:pointer">Delivery</label></td>
      </tr>
    </table>
    <table width="550" border="0" align="center" cellpadding="5" cellspacing="1" class="entryTable">
      <tr>
        <td width="150" class="entryTableHeader">Payment Method </td>
        <td class="content">
        <input name="optPayment" type="radio" id="optPaypal" value="paypal" checked="checked"/>
        <label for="optPaypal" style="cursor:pointer">Paypal</label>
        <input name="optPayment" type="radio" value="cod" id="optCod"  />
        <label for="optCod" style="cursor:pointer">Cash on Delivery</label></td>
      </tr>
    </table>

 

The error I get is document.getElementById(...) is null

 

where am i going wrong?

[Irate]

Check again.

 

First you refer to the id as optDelivery, but in your HTML it is named optDel.

[White_Lily]

and to add to irates answer, you may also want to check the comparison of the value in the input field to, as you stated if(document.getElementById('optDelivery').value == 'optDel') when in actual fact it is (and always will be unless you change it elsewhere) equal to Delivery not optDel.