How to grey out checkbox unless radio button is selected?

Question

very new to javascript, but any help to get me started would be appreciated. I have a simple form:

<div><input type="radio" name="o1" id="burger" />Burger</div>
<div id="yesFries"><input type="checkbox" name="e1" id="fries" />Fries with that?</div>
<div><input type="radio" name="o1" id="pizza" />Pizza</div>
<div><input type="radio" name="o1" id="hotdog" />Hot Dog</div>

I want the "Fries" checkbox greyed out unless the "Burger" radio button is selected. I'm not sure if Javascript or CSS is the best way to do it. Thanks in advance!

Answer 1

I've noticed that you don't specify whether or not you can use jQuery. If that's an option, please see one of the other posts as I highly recommend it.

If you cannot use jquery then try the following:

<script>
  function setFries(){
    var el = document.getElementById("burger");
    if(el.checked)
      document.getElementById("fries").disabled = false;
    else
     document.getElementById("fries").disabled = true;    
  }  
</script>


<div><input type="radio" name="o1" id="burger" onchange="setFries();"/>Burger</div>
<div id="yesFries"><input type="checkbox" name="e1" id="fries" disabled="disabled"/>Fries with that?</div>
<div><input type="radio" name="o1" id="pizza" onchange="setFries();"/>Pizza</div>
<div><input type="radio" name="o1" id="hotdog" onchange="setFries();"/>Hot Dog</div>​

Simple example on jsFiddle