title: JS: Change select option value on being selected with prompt?
body: <p>I've looked around and I don't see this being asked before.</p>
  
  <p>I have a select box, like so:</p>
  
  <pre><code>&lt;select onchange="change()"&gt;
    &lt;option value="" selected&gt;Option 1&lt;/option&gt;
    &lt;option value="30"&gt;Option 2&lt;/option&gt;
    &lt;option value="90"&gt;Option 3&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>I want to add another option...</p>
  
  <pre><code>  &lt;option value="custom"&gt;Option 4&lt;/option&gt;
  </code></pre>
  
  <p>...that when chosen (clicked) an alert box will popup asking the user to type in a number (in the case 30 or 90 weren't viable options, as in the values of the option's) to replace the value of the option.</p>
  
  <pre><code>&lt;script&gt;
  function change() {
    if(value == "custom") {
      value = prompt("Please enter a new number:", "60");
    }
  }
  &lt;/script&gt;
  </code></pre>
  
  <p>I wanted to know what the best way to do this is with plain old javascript - I'll use jQuery if I have to.</p>
  
  <p>Any ideas? An example would be great as well.</p>