title: Button disable unless both input fields have value
body: <p>I have a form with two input fields and a submit button on my page, I would like to have the feature that the 'submit' button is <strong>disabled</strong> until there are values on both two input fields. That's the button will be clickable if and only if there are values input in both fields.</p>
  <p>How to implement this with js and jQuery?</p>
  <p>Here is my page:</p>
  <pre><code>&lt;html&gt;
  &lt;body&gt;
      &lt;form method=post&gt;
          &lt;input type=text id='first_name'&gt;
          &lt;input type=text id='second_name'&gt;
          &lt;input type=submit value=Submit&gt;
      &lt;/form&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  <p>I would like to have both js and jQuery solution</p>