Question

I have this form:

<input type="checkbox" value="First" />First
<input type="checkbox" value="Second" />Second
<input type="checkbox" value="Third" />Third

<select name="first-drop">
    <option value="---">---</option>
    <option value="Option1">Option1</option>
    <option value="Option2">Option2</option>
</select>

​Is there a way (using JS/jQuery) I can change the value of first-drop to --- whenever the First check box is unchecked?

Answer 1

$('input[type="checkbox"][value="First"]').change(function (){
    $('select[name="first-drop"]').toggle();
});​

LIVE DEMO

Note that I fixed you markup:

<input type="checkbox" value="First" checked="checked" >First
<input type="checkbox" value="Second" />Second
<input type="checkbox" value="Third" />Third

<select name="first-drop">
    <option value="">---</option>
    <option value="Option1">Option1</option>
    <option value="Option2">Option2</option>
</select>​

---

Update:

What you want now is:

$('input[type="checkbox"][value="First"]').change(function() {
    if (!this.checked) {
        $('select[name="first-drop"] option:first').prop('selected', 'selected');
    }
});

​LIVE DEMO

If you will give your DOM elements ids it can be simple as that:

$('#first').change(function() {
    if (!this.checked) 
        $('#nothing').prop('selected', 'selected');
});

Updated DOM:

<input type="checkbox" value="First" id="first" >First
<input type="checkbox" value="Second">Second
<input type="checkbox" value="Third">Third

<select name="first-drop">
    <option value="" id="nothing" >---</option>
    <option value="Option1" >Option1</option>
    <option value="Option2">Option2</option>
</select>​

LIVE DEMO